[Low Priority] [Remote Agents] A2A Machine-to-Machine Auth

**Note: Low Priority.** Gemini CLI is primarily run in an interactive environment. This issue tracks non-interactive, service-to-service authentication mechanisms.

**Connection Mechanisms:**
- **OAuth 2.0 Client Credentials Flow:**
- The CLI acts as its own "user" using a `client_id` and `client_secret`.
- Tokens are retrieved directly from the `tokenUrl` without browser interaction.
- **Mutual TLS (mTLS):**
- Support transport-layer authentication using client certificates.
- Allow users to specify certificate and key paths in their `AgentDefinition`.

**Requirements:**
- Implement service-to-service authentication logic.
- Allow configuration of M2M credentials via `AgentDefinition`.