Oauth integration

[JannisNoordmann]

This PR introduces comprehensive OAuth 2.0 support through the new OAuthHelper and improves the repository's security, fault-tolerance, and internal test coverage.

Key changes include:

• OAuth 2.0 Implementation: Added a fully-fledged OAuthHelper providing methods to build authorization URLs, exchange codes, auto-refresh tokens when encountering HTTP 401, and revoke tokens locally as well as remotely. Includes state checking against CSRF.
• Decoupled Token Storage: Introduced TokenStorageInterface (with a default MemoryTokenStorage) so multi-tenant applications can persist OAuth tokens on a per-user basis (e.g., in a database).
• Coverage & Testing: Improving coverage
• Documentation Refactoring: Rewrite of the [README.md] to cleanly separate Static Tokens vs OAuth 2.0 Flows. Added detailed examples for CSRF-protected Callbacks and properly injecting DB-based TokenStorageInterfaces.