Source Google Analytics: Support webflow Oauth

[sherifnada]

Tell us about the problem you're trying to solve

We currently only support json_key authentication to Google Analytics. However, with the release of Airbyte Cloud, we need to start supporting Oauth, the more common way of authenticating to GA.

Instead of the current credentials_json field the GA config should accept one of:

1. credentials_json
2. oauth <-- this one should accept client_id, client_secret, and refresh token

This should be a oneof nested inside a top-level field called "auth_mechanism" . See the connector spec reference in the docs for reference on how this can be implemented.

This should be done in a backwards compatible manner i.e: users currently supplying the json key in the config's top-level should not be impacted by this change.