feat: Add extension methods for simplyfying A2AClient usage

[Blackhex]

Adds utility extension methods to A2AClient abstracting usage of "transport" DTOs.

Implements #132.

[gemini-code-assist]

Summary of Changes

Hello @Blackhex, 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 introduces a set of utility extension methods for the A2AClient class. The primary goal is to simplify the client's API by abstracting the underlying 'transport' Data Transfer Objects (DTOs). This change allows developers to interact with the A2AClient methods using more direct and intuitive parameter lists, reducing boilerplate code and improving usability. The new methods cover common operations like sending messages, canceling tasks, and managing push notifications, making the A2AClient more convenient for various use-cases.

Highlights

• New Extension Methods for A2AClient: Introduces a new static class A2AClientExtensions which provides a more convenient and simplified API for interacting with the A2AClient by abstracting the direct usage of 'transport' DTOs.
• Simplified SendMessageAsync: An overload for SendMessageAsync is added, allowing direct passing of Message, MessageSendConfiguration, and metadata without the need to explicitly construct a MessageSendParams object.
• Simplified CancelTaskAsync: An overload for CancelTaskAsync is added, enabling task cancellation by directly providing the taskId and optional metadata instead of requiring a TaskIdParams object.
• Simplified SetPushNotificationAsync: An overload for SetPushNotificationAsync is introduced, streamlining the configuration of push notifications by directly accepting taskId, url, and other notification parameters.
• Simplified GetPushNotificationAsync: An overload for GetPushNotificationAsync is added, allowing retrieval of push notification configurations by directly providing the taskId and optional pushNotificationConfigId.
• Simplified SendMessageStreamingAsync: An overload for SendMessageStreamingAsync is introduced, simplifying the process of sending streaming messages by directly accepting Message, MessageSendConfiguration, and metadata.

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[gemini-code-assist]

Code Review

This pull request introduces a set of useful extension methods for A2AClient, which simplifies its usage by abstracting away the transport DTOs. The implementation is clean and follows a good pattern. My feedback focuses on improving robustness by adding argument validation, which is a common practice in the existing codebase and will improve the developer experience.