feat: support authentication on server side

[guglielmo-san]

Description

This PR integrates a support for authentication on the server side.
A new User interface is created to represent the Authenticated and UnAuthenticated user in the context, with 2 required function:

1. isAuthenticathed()
2. userName()

The callback authenticathedUserExtractor provided to the A2AExpressApp is responsible for extracting the user object from the Request (if set up by the authentication middleware), and returning an instance of the User interface. The returned instance will be added to the ServerCallContext.
If no authentication middleware or no authenticathedUserExtractor is provided to the Server, the user in the ServerCallContext will be of type UnAuthenticated.

Fixes #194 🦕

Release-As: 0.3.6

[gemini-code-assist]

Summary of Changes

Hello @guglielmo-san, 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 significantly enhances the server's security posture by introducing support for basic bearer token authentication. By integrating industry-standard libraries like express-openapi-validator and passport, it ensures that API requests are properly authenticated against defined security schemes. This change provides a foundational layer for securing server-side operations and managing user access based on token validity.

Highlights

• Server-Side Bearer Authentication: Implemented basic bearer token authentication on the server side to secure API endpoints. This feature leverages OpenAPI 3.0 definitions for security analysis.
• OpenAPI Validator Integration: Integrated express-openapi-validator to analyze security and security schemes based on the OpenAPI 3.0 definition, ensuring requests conform to defined security policies.
• Passport.js for Token Verification: Utilized the passport plugin with a JwtStrategy to verify the validity of bearer tokens, allowing for robust authentication flows.
• User Context Enhancement: Introduced User, AuthenticatedUser, and unAuthenticatedUser types to enrich the ServerCallContext, providing clear authentication status for incoming requests.
• Dynamic OpenAPI Spec Generation: The A2AExpressApp now dynamically generates an OpenAPI specification based on the agent's AgentCard security definitions, which is then used by the validator middleware.

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[gemini-code-assist]

Code Review

This pull request introduces bearer token authentication, which is a great addition. The implementation using express-openapi-validator and passport is mostly solid and includes good test coverage. However, I've identified a critical security vulnerability with a hardcoded JWT secret that must be addressed. I've also provided suggestions to improve efficiency by removing unnecessary file I/O, to enhance functionality by preserving user data from the JWT payload, and to align with common naming conventions. Overall, these are valuable changes that will be even better with these improvements.

[guglielmo-san]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a solid implementation for basic bearer authentication on the server side. The changes are well-structured, with a clear separation of concerns by introducing an A2AUser interface and related classes to abstract user information. The ServerCallContext is appropriately updated to carry user details, and the JSON-RPC handler is cleanly modified to build this context from the incoming request. The accompanying tests effectively validate the new functionality for both authenticated and unauthenticated scenarios. My feedback focuses on enhancing the robustness of the ProxyUser class to ensure type safety when dealing with user objects from various authentication middlewares.