Oauth2 Support

[isc-klu]

Refactor authentication handling to support OAuth2 and other credential types beyond basic auth. Extract shared protocol types to a new common package for better type safety across client/server boundaries.

Key changes:

• Introduce common package with shared ServerSpec and protocol method types
• Replace password-based auth with flexible credentials field supporting both basic auth and OAuth2 bearer tokens
• Add type-safe request/response handlers in client using ProtocolMethods interface
• Update REST client to use credential objects instead of direct username/password
• Why: This architecture enables the extension to support modern authentication patterns (OAuth2, bearer tokens, API keys) while maintaining backward compatibility with existing password-based workflows.