feat: add support for SLIMRPC custom protocol binding

[Tehsmash]

Summary

Add support for the SLIMRPC custom protocol binding as an optional transport in the A2A CLI, selectable via --transport slimrpc.

Background

SLIMRPC is Protobuf RPC over SLIM (Secure Low-Latency Interactive Messaging) — a transport layer designed for agentic workloads that provides:

• Peer-to-peer architecture with NAT/firewall traversal (no centralized broker)
• Hierarchical name-based addressing (org/namespace/service) instead of URLs
• End-to-end encryption via MLS
• Native multicast and group communication
• Bidirectional low-latency messaging

The protocol binding identifier is https://a2a-protocol.org/bindings/experimental-slimrpc/v1. Reference Go implementation: github.com/agntcy/slim-a2a-go.

Proposed Changes

• Add slimrpc to the --transport flag (alongside rest, jsonrpc, grpc)
• Add SLIM-specific flags:
  • --slim-endpoint — SLIM node endpoint (default http://127.0.0.1:46357)
  • --slim-local-name — local SLIM identity (org/namespace/app, default agntcy/a2a-cli/client)
  • --slim-secret — shared secret for app authentication (optional)
• CGO-gated implementation: the SLIM bindings require CGO (github.com/agntcy/slim-bindings-go/v2); a stub with a clear error message is compiled when CGO_ENABLED=0
• CI: run slim-bindings-setup to install the native library before build/test

Usage

a2a send --transport slimrpc --endpoint agntcy/demo/echo_agent \
  --slim-endpoint http://127.0.0.1:46357 \
  --slim-secret my_secret \
  "hello"

References

• Spec: https://github.com/a2aproject/experimental-cpb-slimrpc
• Go library: https://github.com/agntcy/slim-a2a-go
• SLIM project: https://github.com/agntcy/slim