Unify client types

[jafermarq]

This PR removes ClientLike usage and therefore everywhere a client object of type Client is expected. For this, the idea is that users will call <their_client>.to_client() when returning inside their ClientFn. to_client() is a new method that both Client and NumPyClient have.

For backwards compatibility:

• the entry point via start_numpy_client when users supply a client object (that should be of type ClientNumPy). In this case, .to_client() is called internally and transparently to the user. However a Deprecation warning is triggered encouraging users to use start_client instead.
• Similarly in spirit, for simulation, we trigger a warning (once) if the type of client returned in client_fn is not of type Client. Then, the client is converted to Client type.

In addition this PR:

• The content of numpy_client_wrapper.py has been moved to client.py (where it used to be not long ago)

Part of the content above was done in #2493.