Merge https://github.com/google/adk-python/pull/2960
1. All in one authentication sample (has an IDP, Agent and the application) under `contributing/samples/authn-adk-all-in-one/`
2. Documented for all the steps.
3. OAuth 2.0 Authorization Code Grant type used by the agent.
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/2960 from nikhilpurwant:main dfcc821602d265c4ae7cc42eb1f5739beaad6f87
PiperOrigin-RevId: 808672120