Feat: Add Go Timestamp Extension Sample

[Iwaniukooo11]

PR Description: Feat: Add Go Timestamp Extension Sample

Overview

This PR introduces a brand-new Go implementation of the Timestamp Extension sample using a2a-go/v2.

The sample demonstrates how to enrich outgoing A2A messages and artifacts with compliance timestamps in a modular, decoupled, and automated manner. Its architecture is aligned with the latest Python sample standards.

Key Features

1. Modular Architecture & Package Structure

   • timestamp_ext/: The core library package.
     • core.go: Houses the main TimestampExtension struct, metadata settings, and functional options (like WithClock for testing).
     • server.go: Implements server-side interceptors (ServerInterceptor) and WrapExecutor to automatically timestamp outgoing messages and artifacts.
     • client.go: Implements ClientInterceptor to inject the extensions header and stamp client messages.
   • Showcase Server (main.go & agent_executor.go): Launches an HTTP JSON-RPC agent server running an echoExecutor that delegates response text generation to an echoAgent struct.
   • Integration Client Tests (client_test.go): Validates the end-to-end client-server round trip, verifying that both message and artifact metadata timestamps are correctly generated and verified.

2. Compliance & Testing Support

   • Supports fixed clock generation (via WithClock) for deterministic end-to-end testing verification.

3. Complete Showcase Documentation

   • Includes a fully detailed README.md containing architectural walkthroughs, usage guides for both server-side and client-side setup, and instructions on running the standalone server and tests.