[FEATURE] Support OAuth authentication in Python client

[sijie]

Is your feature request related to a problem? Please describe.

The Python Client doesn't support OAuth2 authentication yet.

Describe the solution you'd like

You can provide an implementation of such a function:

def _get_token():
   # TODO: return a token and the expire time

and pass this function to oauth_cb. See example: https://github.com/confluentinc/confluent-kafka-python/blob/master/examples/oauth_producer.py#L32

Describe alternatives you've considered

N/A

Additional context

N/A