[Cleanup]: 1.0 breaking changes

[ishymko]

Umbrella issue to collect all scheduled API breaking changes which can be done with 1.0 release

• A2AClient - this client is built around JSON-RPC types and can't be used in transport-agnostic way. New transport agnostic client can be renamed to A2AClient or this one can just be removed.
• JsonRpcTransport - remove all the code created for compatibility with the old A2AClient client.
• ServerCallContext - the parameter should be made mandatory in all extension points.
• A2AExpressApp - should be removed in favor of using middlewares directly.