Create TCK tests for early hints

[markt-asf]

I've taken an initial look at this and it appears that the JRE provided HttpClient doesn't understand early hints. Neither does Apache HttpClient.

If we limit the test to looking for:

• an interim response with 103 status code
• one or more expected headers with expected values
• ignore all other headers
• a final response with a 200 status code
• one or more expected headers with expected values
• possibly ensure one or more headers are not present
• ignore all other headers

I think we could do this with a relatively simple client that writes/reads directly to/from the socket. The risk is that there is sufficient variation in the responses that the client ends up being more complex. I think we avoid most/all of this by only considering specific headers that we set (and unset?) in the test.

If there are some utilities in the TCK we can re-use to do most/all of this that would be great but I haven't found any so far.

Thoughts?

[IamLRBA]

@markt-asf your approach seems like a pragmatic way to test early hints without relying on external libraries. I'd be happy to assist in exploring the socket-based client implementation or further scoping the test cases. Let me know how I can help!