Securely ingest private GitHub content using GitHub App authentication

Knowledge sources now support GitHub App authentication (app_id, installation_id, private_key) in addition to personal access tokens. This gives platform and security teams a more governed authentication path, with scoped permissions, no personal credentials in the pipeline, and thread-safe token caching that handles expiration automatically. Both sync and async variants are supported.

Details**:**

• Authenticate as a GitHub App for fine-grained, org-managed access to private repositories
• Thread-safe token caching eliminates redundant auth requests and simplifies concurrent workloads
• Personal access tokens continue to work; no migration required

See cookbook for reference