HttpClient: Provide access to the whole set of headers that were sent over the wire

HttpResponse<?>.request().headers() (or some new API point) should provide access to the whole set of headers that were sent over the wire to help for the implementation of caching policies.

See: https://mail.openjdk.java.net/pipermail/net-dev/2021-May/015922.html

Note that the full set of headers is protocol dependent and usually not known until the request is actually sent.