Feat: Add JS Timestamp Extension Sample

[Iwaniukooo11]

PR Description: Feat: Add JS/TS Timestamp Extension Sample

Overview

This PR introduces a brand-new JavaScript/TypeScript implementation of the Timestamp Extension sample using @a2a-js/sdk v1.0.

The sample demonstrates how to enrich outgoing A2A messages and artifacts with compliance timestamps in a modular, decoupled, and automated manner.

Key Features

1. Modular Architecture & Package Structure

   • timestamp_ext/: The core library package.
     • core.ts: Houses the main TimestampExtension helper class and metadata settings.
     • server.ts: Houses the wrapExecutor function to automatically timestamp outgoing events.
     • client.ts: Houses client-side interceptors (wrapClientFactory and clientInterceptor) to inject headers and stamp outgoing client messages.
   • Showcase Server (index.ts & agent_executor.ts): Launches an Express JSON-RPC agent server running an EchoExecutor that delegates response text generation to an EchoAgent class.
   • Integration Client Tests (test_client.ts): 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 a custom clock callback) 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.