[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.

grpc/src/compiler/BUILD.bazel

@@ -95,6 +95,8 @@ cc_library(
     visibility = ["//visibility:private"],
     deps = [
         "//:flatbuffers",
+        "//include/codegen:namer",
+        "//include/codegen:python",
     ],
 )