Add support for features added in 0.2.4 - Extensions, transports

https://github.com/a2aproject/A2A/compare/v0.2.3...v0.2.4

To do:
- Add 0.2.4 features
- Update protocol version in agent card

Review the diff between v0.2.3 and v0.2.4 in the A2A repository to understand the new extensions and transports features. Update the protocol version in the agent card, likely in a configuration or constants file. Look for existing transport and extension implementations to see how to integrate the new features.

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