fix: Adding check for invalid Client instantiation

[GuilhermeFreire]

This solves #145
I just added a check and error message in case the user attempts to instantiate the Client improperly.

Now this is what happens if you instantiate a Client with no arguments and no env var set:

>>> import label_studio_sdk
>>> label_studio_sdk.Client()

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/to/label_studio_sdk/client.py", line 90, in __init__
    raise RuntimeError(
RuntimeError: If neither 'api_key' nor 'credentials' are provided, 'LABEL_STUDIO_API_KEY' environment variable must be set