Implement Serialize on generated rust types (#7022)

* fix for rust build

* Rust: Implement Serialize on generated types

For debugging convenience it is really handy to be able to dump out
types as another format (ie: json). For example, if we are logging a
type to a structured logging system, or even printing it out in a
structured way to the console.

Right now we handle this by shelling out to `flatc` which is not ideal;
by implementing Serialize on the generated types we can use any of the
Serializer-implementing packages for our structured debug output.

* clang-format

* Make the flatbuffers Rust crate only have an optional dependency on the `serde` packages.

* fix warning

* fix rust test build

* Oh yeah this needs to be initialized

* fix toml syntax

* code review feedback

* rebuild test data

src/flatc.cpp

@@ -184,6 +184,7 @@ const static FlatCOption options[] = {
   { "", "reflect-types", "",
     "Add minimal type reflection to code generation." },
   { "", "reflect-names", "", "Add minimal type/name reflection." },
+  { "", "rust-serialize", "", "Implement serde::Serialize on generated Rust types." },
   { "", "root-type", "T", "Select or override the default root_type." },
   { "", "require-explicit-ids", "",
     "When parsing schemas, require explicit ids (id: x)." },
@@ -469,6 +470,8 @@ int FlatCompiler::Compile(int argc, const char **argv) {
         opts.mini_reflect = IDLOptions::kTypes;
       } else if (arg == "--reflect-names") {
         opts.mini_reflect = IDLOptions::kTypesAndNames;
+      } else if (arg == "--rust-serialize") {
+        opts.rust_serialize = true;
       } else if (arg == "--require-explicit-ids") {
         opts.require_explicit_ids = true;
       } else if (arg == "--root-type") {