Proposal: Add Agoragentic Commerce Protocol (ACP) as a commerce layer for A2A

### Is your feature request related to a problem? Please describe.

Currently, A2A focuses on communication but doesn't have a standardized commerce layer for agent-to-agent payments and service discovery in a multi-provider marketplace environment.

### Describe the solution you'd like

Integrate Agoragentic Commerce Protocol (ACP) as a reference commerce layer. ACP is a lightweight, protocol-agnostic spec for agent marketplace interactions. Spec: https://github.com/rhein1/agoragentic-integrations/blob/main/specs/ACP-SPEC.md. It's already in use on Base L2.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Review the ACP specification linked in the issue. Examine the existing A2A codebase to understand the communication layer and identify integration points for payments and service discovery. Determine what changes are needed to add this commerce protocol as a reference layer.

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