feat: add signing and verifying example used to demonstrate AgentCard signing and verifying

[sokoliva]

This change introduces a new agent whose agent_card is signed on the server side using signer function created by create_agent_card_signer and verified on the client side using a signature_verifier function created by create_signature_verifier.

Relevant link: Agent Card Signing

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[gemini-code-assist]

Summary of Changes

Hello @sokoliva, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request delivers a robust example of an Agent-to-Agent (A2A) agent that integrates AgentCard signing and verification. The primary goal is to illustrate how agents can establish trust by cryptographically signing their identity cards, which clients can then verify. This example covers the full lifecycle from key generation and server-side signing of both public and extended agent cards, to client-side fetching and validation of these signatures, thereby enhancing the security posture of A2A interactions.

Highlights

• New Signed Agent Example: Introduces a complete example of an A2A agent that demonstrates the signing and verification of AgentCards, showcasing secure communication practices.
• Server-Side AgentCard Signing: The agent generates an EC key pair and uses the private key to sign both its public and authenticated extended AgentCards on the server side.
• Client-Side Signature Verification: A dedicated test client is provided to fetch the AgentCards and verify their signatures using the corresponding public key, ensuring card authenticity.
• Containerization Support: Includes a Containerfile for easily building and running the signed agent within a container environment, simplifying deployment and testing.
• Comprehensive Documentation: A README.md file details how to set up, run, build, and validate the example, along with an important disclaimer regarding untrusted input in production.

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[gemini-code-assist]

Code Review

This pull request introduces a valuable example demonstrating agent card signing and verification. The implementation is well-structured, including a Containerfile and a comprehensive README.md. My review focuses on improving dependency management, container security, and adherence to standards like RFC 7515 for JWTs. I've also included minor suggestions for code style and error handling to enhance the quality of the example.