[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.fbs

@@ -0,0 +1,11 @@
+namespace example;
+
+table HelloRequest {}
+table HelloResponse {}
+
+rpc_service HelloService {
+  Hello(HelloRequest):HelloResponse;
+  StreamClient(HelloRequest):HelloResponse (streaming: "client");
+  StreamServer(HelloRequest):HelloResponse (streaming: "server");
+  Stream(HelloRequest):HelloResponse (streaming: "bidi");
+}