feat(rest, jsonrpc): Add client-side extension support

[sokoliva]

feat(rest, jsonrpc): Add client-side extension support
This commit introduces support for clients to declare the extensions they support.

• Adds an extensions list to ClientConfig.
• Updates ClientFactory to pass client_extensions to JsonRpcTransport and RestTransport.
• Adds _update_extension_header method to both transports to update the X-A2A-Extensions header.
• Modifies send_message and send_message_streaming in JsonRpcTransport to include the extension headers.
• Modifies _prepare_send_message in RestTransport to include the extension headers.
• Adds tests for the extension header logic in both JSON-RPC and REST transports, including a new test file test_rest_client.py.

Fixes #504 🦕

[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 introduces a new feature that allows clients to explicitly declare the extensions they support. By adding an "extensions" field to the client configuration and propagating this information through a dedicated HTTP header ("X-A2A-Extensions") in both JSON-RPC and REST communications, the system gains a standardized mechanism for clients to signal their capabilities to the server. This enhancement improves interoperability and enables more sophisticated client-server interactions based on supported extensions.

Highlights

• Client Configuration: Introduced an "extensions" field in "ClientConfig" to allow clients to declare supported extensions.
• Transport Integration: Updated "ClientFactory" to pass these extensions to both "JsonRpcTransport" and "RestTransport" during their initialization.
• Header Management: Implemented a new "_update_extension_header" method in both transport classes to populate the "X-A2A-Extensions" HTTP header with the declared extensions.
• Message Sending: Modified "send_message" and "send_message_streaming" methods in both JSON-RPC and REST transports to include the "X-A2A-Extensions" header in outgoing requests.
• Comprehensive Testing: Added extensive unit tests for the new extension header logic in both "JsonRpcTransport" and "RestTransport", including a new test file for REST client tests.

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[gemini-code-assist]

Code Review

This pull request adds support for clients to declare supported extensions, which are then sent in the X-A2A-Extensions header. The implementation is straightforward, adding the necessary logic to ClientConfig, ClientFactory, and both JsonRpcTransport and RestTransport. The changes are well-tested. My review includes a few suggestions to improve robustness and maintainability. Specifically, I've identified a minor bug in the header parsing logic and an opportunity to reduce code duplication between the two transport implementations. I've also suggested improvements to the tests to cover more edge cases.

[lkawka]

Two things:

• gRPC implementation seems to be missing.
• We would like to have per-method overrides on the Client interface.