Add GitHub OAuth lambda function

[lusloher]

Add another lambda function which enables the frontend to do authentication.

Acceptance Criteria

• unblocking the frontend teams authentication work by adding an auth lambda function

• [x] add a new API endpoint

• [x] integrate with the existing API endpoint

[lusloher]

Link to subfeature: https://jinaai.productboard.com/feature-board/planning/features/7508873