[Feature]: add Google oauth provider connection

Feature Description

Currently Open Notebook allows connecting to LLM providers via API keys. I would like to be able to connect to my Google Pro account using OAuth.

Of course, this applies to several other providers as well including OpenAI, Anthropic, Z.AI GLM Coding Plan etc

We could use openauth + plugins like jenslys/opencode-gemini-auth

Why would this be helpful?

Connecting via an API key is pay-as-you-go. Many providers now offer monthly subscription plans that are more cost-effective.

Proposed Solution (Optional)

This could be implemented using openauth + plugins like jenslys/opencode-gemini-auth

Additional Context

No response

Contribution

• I am a developer and would like to work on implementing this feature (pending maintainer approval)