[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.

src/BUILD.bazel

@@ -89,6 +89,7 @@ cc_library(
     visibility = ["//:__pkg__"],
     deps = [
         ":flatbuffers",
+        "//include/codegen:namer",
     ],
 )
 
@@ -155,6 +156,7 @@ cc_library(
         "//grpc/src/compiler:python_generator",
         "//grpc/src/compiler:swift_generator",
         "//grpc/src/compiler:ts_generator",
+        "//include/codegen:namer",
         "//include/codegen:python",
     ],
 )