[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_generated.pyi

@@ -0,0 +1,26 @@
+from __future__ import annotations
+
+import flatbuffers
+
+import flatbuffers
+import typing
+
+uoffset: typing.TypeAlias = flatbuffers.number_types.UOffsetTFlags.py_type
+
+class HelloRequest(object):
+  @classmethod
+  def GetRootAs(cls, buf: bytes, offset: int) -> HelloRequest: ...
+  @classmethod
+  def GetRootAsHelloRequest(cls, buf: bytes, offset: int) -> HelloRequest: ...
+  def Init(self, buf: bytes, pos: int) -> None: ...
+def HelloRequestStart(builder: flatbuffers.Builder) -> None: ...
+def HelloRequestEnd(builder: flatbuffers.Builder) -> uoffset: ...
+class HelloResponse(object):
+  @classmethod
+  def GetRootAs(cls, buf: bytes, offset: int) -> HelloResponse: ...
+  @classmethod
+  def GetRootAsHelloResponse(cls, buf: bytes, offset: int) -> HelloResponse: ...
+  def Init(self, buf: bytes, pos: int) -> None: ...
+def HelloResponseStart(builder: flatbuffers.Builder) -> None: ...
+def HelloResponseEnd(builder: flatbuffers.Builder) -> uoffset: ...
+