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_

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