[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.py

@@ -0,0 +1,97 @@
+# Generated by the gRPC FlatBuffers compiler. DO NOT EDIT!
+
+import flatbuffers
+import grpc
+
+from service_test_generated import HelloRequest, HelloResponse
+
+
+def _serialize_to_bytes(table):
+  buf = table._tab.Bytes
+  n = flatbuffers.encode.Get(flatbuffers.packer.uoffset, buf, 0)
+  if table._tab.Pos != n:
+    raise ValueError('must be a top-level table')
+  return bytes(buf)
+
+
+class HelloServiceStub(object):
+  '''Interface exported by the server.'''
+
+  def __init__(self, channel):
+    '''Constructor.
+
+    Args:
+      channel: A grpc.Channel.
+    '''
+
+    self.Hello = channel.unary_unary(
+      method='/example.HelloService/Hello',
+      request_serializer=_serialize_to_bytes,
+      response_deserializer=HelloResponse.GetRootAs)
+
+    self.StreamClient = channel.stream_unary(
+      method='/example.HelloService/StreamClient',
+      request_serializer=_serialize_to_bytes,
+      response_deserializer=HelloResponse.GetRootAs)
+
+    self.StreamServer = channel.unary_stream(
+      method='/example.HelloService/StreamServer',
+      request_serializer=_serialize_to_bytes,
+      response_deserializer=HelloResponse.GetRootAs)
+
+    self.Stream = channel.stream_stream(
+      method='/example.HelloService/Stream',
+      request_serializer=_serialize_to_bytes,
+      response_deserializer=HelloResponse.GetRootAs)
+
+
+class HelloServiceServicer(object):
+  '''Interface exported by the server.'''
+
+  def Hello(self, request, context):
+    context.set_code(grpc.StatusCode.UNIMPLEMENTED)
+    context.set_details('Method not implemented!')
+    raise NotImplementedError('Method not implemented!')
+
+  def StreamClient(self, request_iterator, context):
+    context.set_code(grpc.StatusCode.UNIMPLEMENTED)
+    context.set_details('Method not implemented!')
+    raise NotImplementedError('Method not implemented!')
+
+  def StreamServer(self, request, context):
+    context.set_code(grpc.StatusCode.UNIMPLEMENTED)
+    context.set_details('Method not implemented!')
+    raise NotImplementedError('Method not implemented!')
+
+  def Stream(self, request_iterator, context):
+    context.set_code(grpc.StatusCode.UNIMPLEMENTED)
+    context.set_details('Method not implemented!')
+    raise NotImplementedError('Method not implemented!')
+
+
+def add_HelloServiceServicer_to_server(servicer, server):
+  rpc_method_handlers = {
+    'Hello': grpc.unary_unary_rpc_method_handler(
+      servicer.Hello,
+      request_deserializer=HelloRequest.GetRootAs,
+      response_serializer=_serialize_to_bytes),
+    'StreamClient': grpc.stream_unary_rpc_method_handler(
+      servicer.StreamClient,
+      request_deserializer=HelloRequest.GetRootAs,
+      response_serializer=_serialize_to_bytes),
+    'StreamServer': grpc.unary_stream_rpc_method_handler(
+      servicer.StreamServer,
+      request_deserializer=HelloRequest.GetRootAs,
+      response_serializer=_serialize_to_bytes),
+    'Stream': grpc.stream_stream_rpc_method_handler(
+      servicer.Stream,
+      request_deserializer=HelloRequest.GetRootAs,
+      response_serializer=_serialize_to_bytes),
+  }
+
+  generic_handler = grpc.method_handlers_generic_handler(
+    'example.HelloService', rpc_method_handlers)
+
+  server.add_generic_rpc_handlers((generic_handler,))
+
+