feat: Support exposing Agents via A2A protocol

- [x] I have looked for existing issues (including closed) about this

## Feature Request

Add support for exposing Rig agents using Google’s new [A2A protocol](https://github.com/google/A2A) to enable standardized interoperability with other agentic ecosystems.

### Motivation

Interoperability between agentic systems is currently fragmented. By enabling Rig to expose agents using the A2A protocol, we can position Rig as a plug-and-play agentic SDK across diverse platforms. This improvement will simplify integrations and reduce the maintenance overhead associated with custom wrappers and integration layers.

### Proposal

After reviewing the A2A specification, I found it to be solid and well-structured. The proposal is to implement native support for the A2A protocol in Rig, allowing developers to expose agents with just a couple of lines of code. Integration could be facilitated via Axum or any other compatible web framework. This approach abstracts away the complexity of interoperability and provides a standardized mechanism that will likely see wide adoption.

### Alternatives

Alternatives include manually developing custom wrappers or integration layers for interoperability. However, those approaches tend to be less standardized and harder to maintain. Adopting the A2A protocol offers a more robust, community-supported solution for long-term sustainability.