Add Cython functionality to directly wrap grpc_arg

src/python/grpcio/grpc/_cython/_cygrpc/arguments.pxd.pxi

@@ -1,4 +1,4 @@
-# Copyright 2018 gRPC authors.
+# Copyright 2018 The gRPC Authors
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -22,6 +22,9 @@ cdef void _destroy_pointer(void* pointer)
 cdef int _compare_pointer(void* first_pointer, void* second_pointer)
 
 
+cdef tuple _wrap_grpc_arg(grpc_arg arg)
+
+
 cdef class _ArgumentProcessor:
 
   cdef grpc_arg c_argument

src/python/grpcio/grpc/_cython/_cygrpc/arguments.pyx.pxi

@@ -34,6 +34,18 @@ cdef int _compare_pointer(void* first_pointer, void* second_pointer):
     return 0
 
 
+cdef class _GrpcArgWrapper:
+
+  cdef grpc_arg arg
+
+
+cdef tuple _wrap_grpc_arg(grpc_arg arg):
+
+  wrapped = _GrpcArgWrapper()
+  wrapped.arg = arg
+  return ("grpc.python._cygrpc._GrpcArgWrapper", wrapped)
+
+
 cdef class _ArgumentProcessor:
 
   cdef void c(self, argument, grpc_arg_pointer_vtable *vtable, references):
@@ -51,6 +63,8 @@ cdef class _ArgumentProcessor:
       if encoded_value is not value:
         references.append(encoded_value)
       self.c_argument.value.string = encoded_value
+    elif isinstance(value, _GrpcArgWrapper):
+      self.c_argument = (<_GrpcArgWrapper>value).arg
     elif hasattr(value, '__int__'):
       # Pointer objects must override __int__() to return
       # the underlying C address (Python ints are word size). The