Fix the return type of client_fn in simulation

[adam-narozniak]

Currently, the simulation requires client_fn of type Callable[[str], Client]; however, it works on both Client and NumpyClient (also, the example on the website show function with the return type of NumpyClient).
This PR changes the type to ClientLike, which is Union[Client, NumPyClient].
The problem was mentioned in #1176

Both type work because the simulation uses RayClientProxy that calls this function

flower/src/py/flwr/client/app.py

Lines 249 to 253 in 57a4365

	def to_client(client_like: ClientLike) -> Client:
	"""Take any Client-like object and return it as a Client."""
	if isinstance(client_like, NumPyClient):
	return _wrap_numpy_client(client=client_like)
	return client_like