feat: support HTTP request cancellation - #1541

Implements C3 from the RPC extensions design, stacked on C2 (#1540).

• Adds connector cancellation hooks to Request and delegates them through WrapperRequest.
• Lets Request.abort/Response.abort cancel an active Netty exchange, including an HTTP/2 stream.
• Reports stream failures as I/O errors, updates the response status, and notifies an asynchronous response callback of post-header failures.
• Tests client cancellation, the resulting response callback, read failure, and server write failure.

Validation: mvnw -B -pl org.restlet -Dtest=HttpTrailersTestCase,Http2FullDuplexTestCase,Http2CancellationTestCase test.