[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/BUILD.bazel

@@ -15,10 +15,25 @@ filegroup(
     visibility = ["//visibility:public"],
 )
 
+cc_library(
+    name = "namer",
+    hdrs = [
+        "idl_namer.h",
+        "namer.h",
+    ],
+    strip_include_prefix = "/include",
+    visibility = ["//:__subpackages__"],
+    deps = ["//:runtime_cc"],
+)
+
 cc_library(
     name = "python",
     srcs = ["python.cc"],
     hdrs = ["python.h"],
     strip_include_prefix = "/include",
-    visibility = ["//src:__subpackages__"],
+    visibility = [
+        "//grpc:__subpackages__",
+        "//src:__subpackages__",
+    ],
+    deps = [":namer"],
 )