[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.py

@@ -0,0 +1,58 @@
+# automatically generated by the FlatBuffers compiler, do not modify
+
+# namespace: example
+
+import flatbuffers
+from typing import Any
+class HelloRequest(object):
+    __slots__ = ['_tab']
+
+    @classmethod
+    def GetRootAs(cls, buf, offset: int = 0):
+        n = flatbuffers.encode.Get(flatbuffers.packer.uoffset, buf, offset)
+        x = HelloRequest()
+        x.Init(buf, n + offset)
+        return x
+
+    @classmethod
+    def GetRootAsHelloRequest(cls, buf, offset=0):
+        """This method is deprecated. Please switch to GetRootAs."""
+        return cls.GetRootAs(buf, offset)
+    # HelloRequest
+    def Init(self, buf: bytes, pos: int):
+        self._tab = flatbuffers.table.Table(buf, pos)
+
+def HelloRequestStart(builder: flatbuffers.Builder):
+    builder.StartObject(0)
+
+def HelloRequestEnd(builder: flatbuffers.Builder) -> int:
+    return builder.EndObject()
+
+
+
+class HelloResponse(object):
+    __slots__ = ['_tab']
+
+    @classmethod
+    def GetRootAs(cls, buf, offset: int = 0):
+        n = flatbuffers.encode.Get(flatbuffers.packer.uoffset, buf, offset)
+        x = HelloResponse()
+        x.Init(buf, n + offset)
+        return x
+
+    @classmethod
+    def GetRootAsHelloResponse(cls, buf, offset=0):
+        """This method is deprecated. Please switch to GetRootAs."""
+        return cls.GetRootAs(buf, offset)
+    # HelloResponse
+    def Init(self, buf: bytes, pos: int):
+        self._tab = flatbuffers.table.Table(buf, pos)
+
+def HelloResponseStart(builder: flatbuffers.Builder):
+    builder.StartObject(0)
+
+def HelloResponseEnd(builder: flatbuffers.Builder) -> int:
+    return builder.EndObject()
+
+
+