[Feat] Implement Client-Side Auth for Remote Agents

[adamfweidman]

What would you like to be added?

Enable gemini-cli to securely connect to and interact with remote Agent2Agent (A2A) compatible agents that require authentication. The A2A protocol delegates authentication to standard HTTP mechanisms .

Why is this needed?

Supported Schemes:

• API Key: Support for custom headers (e.g. MY-API-Key: ).
• Bearer Token: Support for JWTs or OAuth tokens.

We must handle full OAuth flow.

Additional context

References

• A2A Protocol: Enterprise Ready (https://a2a-protocol.org/latest/topics/enterprise-ready/)
• A2A Protocol: Specification (https://a2a-protocol.org/latest/specification/)