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.

Review the A2A protocol specification at the linked GitHub repository. Examine Rig's existing agent exposure mechanisms and web framework integration (likely Axum). Look for modules handling agent interfaces and HTTP endpoints. The goal is to implement a new module or extend existing ones to conform to the A2A protocol, allowing agents to be exposed with minimal configuration.

Scritto dal modello di indicizzazione a partire dal testo della issue.