Adds oauth flow to support chat apps to connect to Rill MCP server in dynamic manner without editing configs or generating auth token beforehand.

Breaking change - rill login will require min this version as we changed the serialization of expires_in in token response from string to int64

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!