[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.

tests/service_test_grpc.fb.pyi

@@ -0,0 +1,27 @@
+# Generated by the gRPC FlatBuffers compiler. DO NOT EDIT!
+
+from __future__ import annotations
+
+import grpc
+import typing
+
+from service_test_generated import HelloRequest, HelloResponse
+
+
+class HelloServiceStub(object):
+  def __init__(self, channel: grpc.Channel) -> None: ...
+  def Hello(self, request: HelloRequest) -> HelloResponse: ...
+  def StreamClient(self, request_iterator: typing.Iterator[HelloRequest]) -> HelloResponse: ...
+  def StreamServer(self, request: HelloRequest) -> typing.Iterator[HelloResponse]: ...
+  def Stream(self, request_iterator: typing.Iterator[HelloRequest]) -> typing.Iterator[HelloResponse]: ...
+
+
+class HelloServiceServicer(object):
+  def Hello(self, request: HelloRequest, context: grpc.ServicerContext) -> HelloResponse: ...
+  def StreamClient(self, request_iterator: typing.Iterator[HelloRequest], context: grpc.ServicerContext) -> HelloResponse: ...
+  def StreamServer(self, request: HelloRequest, context: grpc.ServicerContext) -> typing.Iterator[HelloResponse]: ...
+  def Stream(self, request_iterator: typing.Iterator[HelloRequest], context: grpc.ServicerContext) -> typing.Iterator[HelloResponse]: ...
+
+
+def add_HelloServiceServicer_to_server(servicer: HelloServiceServicer, server: grpc.Server) -> None: ...
+