Feat: API keys for Python client authentication

[danielpeng1]

This PR introduces the following:

• Replaced the current username/password authentication with explicit API key authentication.
• Updated the client code to remove the multi-layer, cascading (external) OAuth clients for Hub connectivity.
• Maintained OAuth authentication for bucket write access.
• Updated the docs.

Notes:

• Implementation is complete, but keeping this as a draft PR as the migration on the Hub needs to be complete to accommodate these changes.
• Lacked workflow scope to edit .github/workflows/test.yml to remove the references to POLARIS_USERNAME/POLARIS_PASSWORD and add POLARIS_API_KEY in-place, so these changes can be made too.