[gRPC] Update the code generator for Python to produce typed handlers (#8326)

* Move `namer.h` and `idl_namer.h` to `include/codegen` so they can be reused from `grpc` dirqectory.

* [gRPC] Update the Python generator to produce typed handlers and Python stubs if requested.

* [gRPC] Document the newly added compiler flags.

include/codegen/idl_namer.h

@@ -0,0 +1,179 @@
+#ifndef FLATBUFFERS_INCLUDE_CODEGEN_IDL_NAMER_H_
+#define FLATBUFFERS_INCLUDE_CODEGEN_IDL_NAMER_H_
+
+#include "codegen/namer.h"
+#include "flatbuffers/idl.h"
+
+namespace flatbuffers {
+
+// Provides Namer capabilities to types defined in the flatbuffers IDL.
+class IdlNamer : public Namer {
+ public:
+  explicit IdlNamer(Config config, std::set<std::string> keywords)
+      : Namer(config, std::move(keywords)) {}
+
+  using Namer::Constant;
+  using Namer::Directories;
+  using Namer::Field;
+  using Namer::File;
+  using Namer::Function;
+  using Namer::Method;
+  using Namer::Namespace;
+  using Namer::NamespacedType;
+  using Namer::ObjectType;
+  using Namer::Type;
+  using Namer::Variable;
+  using Namer::Variant;
+
+  std::string Constant(const FieldDef &d) const { return Constant(d.name); }
+
+  // Types are always structs or enums so we can only expose these two
+  // overloads.
+  std::string Type(const StructDef &d) const { return Type(d.name); }
+  std::string Type(const EnumDef &d) const { return Type(d.name); }
+
+  std::string Function(const Definition &s) const { return Function(s.name); }
+  std::string Function(const std::string& prefix, const Definition &s) const {
+    return Function(prefix + s.name);
+  }
+
+  std::string Field(const FieldDef &s) const { return Field(s.name); }
+  std::string Field(const FieldDef &d, const std::string &s) const {
+    return Field(d.name + "_" + s);
+  }
+
+  std::string Variable(const FieldDef &s) const { return Variable(s.name); }
+
+  std::string Variable(const StructDef &s) const { return Variable(s.name); }
+
+  std::string Variant(const EnumVal &s) const { return Variant(s.name); }
+
+  std::string EnumVariant(const EnumDef &e, const EnumVal &v) const {
+    return Type(e) + config_.enum_variant_seperator + Variant(v);
+  }
+
+  std::string ObjectType(const StructDef &d) const {
+    return ObjectType(d.name);
+  }
+  std::string ObjectType(const EnumDef &d) const { return ObjectType(d.name); }
+
+  std::string Method(const FieldDef &d, const std::string &suffix) const {
+    return Method(d.name, suffix);
+  }
+  std::string Method(const std::string &prefix, const StructDef &d) const {
+    return Method(prefix, d.name);
+  }
+  std::string Method(const std::string &prefix, const FieldDef &d) const {
+    return Method(prefix, d.name);
+  }
+  std::string Method(const std::string &prefix, const FieldDef &d,
+                     const std::string &suffix) const {
+    return Method(prefix, d.name, suffix);
+  }
+
+  std::string Namespace(const struct Namespace &ns) const {
+    return Namespace(ns.components);
+  }
+
+  std::string NamespacedEnumVariant(const EnumDef &e, const EnumVal &v) const {
+    return NamespacedString(e.defined_namespace, EnumVariant(e, v));
+  }
+
+  std::string NamespacedType(const Definition &def) const {
+    return NamespacedString(def.defined_namespace, Type(def.name));
+  }
+
+  std::string NamespacedObjectType(const Definition &def) const {
+    return NamespacedString(def.defined_namespace, ObjectType(def.name));
+  }
+
+  std::string Directories(const struct Namespace &ns,
+                          SkipDir skips = SkipDir::None,
+                          Case input_case = Case::kUpperCamel) const {
+    return Directories(ns.components, skips, input_case);
+  }
+
+  // Legacy fields do not really follow the usual config and should be
+  // considered for deprecation.
+
+  std::string LegacyRustNativeVariant(const EnumVal &v) const {
+    return ConvertCase(EscapeKeyword(v.name), Case::kUpperCamel);
+  }
+
+  std::string LegacyRustFieldOffsetName(const FieldDef &field) const {
+    return "VT_" + ConvertCase(EscapeKeyword(field.name), Case::kAllUpper);
+  }
+    std::string LegacyRustUnionTypeOffsetName(const FieldDef &field) const {
+    return "VT_" + ConvertCase(EscapeKeyword(field.name + "_type"), Case::kAllUpper);
+  }
+
+
+  std::string LegacySwiftVariant(const EnumVal &ev) const {
+    auto name = ev.name;
+    if (isupper(name.front())) {
+      std::transform(name.begin(), name.end(), name.begin(), CharToLower);
+    }
+    return EscapeKeyword(ConvertCase(name, Case::kLowerCamel));
+  }
+
+  // Also used by Kotlin, lol.
+  std::string LegacyJavaMethod2(const std::string &prefix, const StructDef &sd,
+                                const std::string &suffix) const {
+    return prefix + sd.name + suffix;
+  }
+
+  std::string LegacyKotlinVariant(EnumVal &ev) const {
+    // Namer assumes the input case is snake case which is wrong...
+    return ConvertCase(EscapeKeyword(ev.name), Case::kLowerCamel);
+  }
+  // Kotlin methods escapes keywords after case conversion but before
+  // prefixing and suffixing.
+  std::string LegacyKotlinMethod(const std::string &prefix, const FieldDef &d,
+                                 const std::string &suffix) const {
+    return prefix + ConvertCase(EscapeKeyword(d.name), Case::kUpperCamel) +
+           suffix;
+  }
+  std::string LegacyKotlinMethod(const std::string &prefix, const StructDef &d,
+                                 const std::string &suffix) const {
+    return prefix + ConvertCase(EscapeKeyword(d.name), Case::kUpperCamel) +
+           suffix;
+  }
+
+  // This is a mix of snake case and keep casing, when Ts should be using
+  // lower camel case.
+  std::string LegacyTsMutateMethod(const FieldDef& d) {
+    return "mutate_" + d.name;
+  }
+
+  std::string LegacyRustUnionTypeMethod(const FieldDef &d) {
+    // assert d is a union
+    // d should convert case but not escape keywords due to historical reasons
+    return ConvertCase(d.name, config_.fields, Case::kLowerCamel) + "_type";
+  }
+
+ private:
+  std::string NamespacedString(const struct Namespace *ns,
+                               const std::string &str) const {
+    std::string ret;
+    if (ns != nullptr) { ret += Namespace(ns->components); }
+    if (!ret.empty()) ret += config_.namespace_seperator;
+    return ret + str;
+  }
+};
+
+// This is a temporary helper function for code generators to call until all
+// flag-overriding logic into flatc.cpp
+inline Namer::Config WithFlagOptions(const Namer::Config &input,
+                                     const IDLOptions &opts,
+                                     const std::string &path) {
+  Namer::Config result = input;
+  result.object_prefix = opts.object_prefix;
+  result.object_suffix = opts.object_suffix;
+  result.output_path = path;
+  result.filename_suffix = opts.filename_suffix;
+  return result;
+}
+
+}  // namespace flatbuffers
+
+#endif  // FLATBUFFERS_INCLUDE_CODEGEN_IDL_NAMER_H_