Add type checking for durable clients

[davidmrdavid]

@vrmdr, @gavin-aguiar: I believe this requires Python worker special support, I recall we had talked about this when GA'ing support for DF in Python prog-model V2. This is because the worker thinks the durable client type is a string, but in reality DF is using that string to construct the object behind the scenes.

[davidmrdavid]

For that reason - should we not move this back to the Python worker repo, @vrdmr? It's not something the DF team can unblock by itself.

[gavin-aguiar]

@davidmrdavid Transferred it back. It will be a change in the python library but we need to validate it with the worker.