[PSM interop] Set hostname in initial metdata for Python server (#29289)

* [PSM interp] Set hostname in initial metdata for Python servers

* DO-NOT-MERGE: hijact psm-security-python for one test case

* Revert "DO-NOT-MERGE: hijact psm-security-python for one test case"

This reverts commit 17b8af9ab9.

* Metadata is a tuple of tuple not a tuple

src/python/grpcio_tests/tests_py3_only/interop/xds_interop_server.py

@@ -59,10 +59,12 @@ class TestService(test_pb2_grpc.TestServiceServicer):
 
     def EmptyCall(self, _: empty_pb2.Empty,
                   context: grpc.ServicerContext) -> empty_pb2.Empty:
+        context.send_initial_metadata((('hostname', self._hostname),))
         return empty_pb2.Empty()
 
     def UnaryCall(self, request: messages_pb2.SimpleRequest,
                   context: grpc.ServicerContext) -> messages_pb2.SimpleResponse:
+        context.send_initial_metadata((('hostname', self._hostname),))
         response = messages_pb2.SimpleResponse()
         response.server_id = self._server_id
         response.hostname = self._hostname