### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### The Feature

**Description:** Currently, when LiteLLM invokes an A2A (Agent-to-Agent) agent, it has limited support for custom headers. While some static headers can be configured via `config.yaml`, there is no flexible way to provide headers that might change or need to be fetched (e.g., rotating API keys, session tokens, or per-user authentication).

**Proposed Solution:** - Add a `headers` or `extra_headers` field to the A2A agent configuration in both `config.yaml` and the Admin UI. - Support environment variable expansion (e.g., `"Authorization": "Bearer os.environ/MY_AGENT_KEY"`) for these headers. - (Optional) Allow a hook or a way to forward specific headers from the incoming client request to the A2A agent (e.g., forwarding an `x-api-key`).

**What part of LiteLLM is this about?** Proxy / A2A Agent Gateway

### Motivation, pitch

**Motivation:** Many agents are hosted behind security gateways or proxy layers (like Apigee, AWS API Gateway, or custom auth services) that require specific headers for authentication. In some cases, these keys are dynami…