feat: add a2a_state_forwarding sample

[Koichi73]

Summary

• Add a sample demonstrating how to forward client-side session.state to a remote ADK agent over the A2A protocol via request metadata
• Client side: a RequestInterceptor copies whitelisted state keys into A2A request metadata
• Server side: a before_agent_callback reads run_config.custom_metadata['a2a_metadata'] and injects the values back into session state so that instruction template placeholders (e.g. {user_name}) resolve correctly

Relates to #3098

Test plan

• Start the remote agent: adk api_server --a2a --port 8001 contributing/samples/a2a_state_forwarding/remote_a2a
• Start the client: adk web contributing/samples/
• Select the a2a_state_forwarding agent and send "greet me" → confirms Hello Alice! How are you doing today? is returned
• Remove user_name from ALLOWED_FORWARD_KEYS → confirms the template remains unresolved (whitelist is effective)

[Koichi73]

Hi @rohityan, thanks for the triage! I noticed the community repo label was added — could you clarify the intent? A couple of concerns about moving this to google/adk-python-community:

1. This PR follows the exact pattern of the existing A2A samples already living in contributing/samples/ (a2a_basic, a2a_auth, a2a_human_in_loop, a2a_root). Splitting one A2A sample off into a separate repo would fragment the discovery experience for users learning A2A.
2. adk-python-community is structured around plugins / services / tools / deployment modules — there isn't a natural home for a sample agent there.

Would you reconsider keeping this here alongside the other A2A samples? If there's a specific concern with the PR I'm happy to address it. Thanks!