Add a test for proxy authentication challenges

Implementors of org.chromium.net.Proxy.Callback should be able to
handle authentication challenges.

Also:
1. Fix the propagation of HttpResponses when using
   NativeTestServer#registerRequestHandler
2. Make the construction of actual RawHttpResponses more ergonomic

Bug: b:441490632
Change-Id: Ief195c9a74851c11bf62adaba3841ac4413154b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6897972
Reviewed-by: Etienne Dechamps <edechamps@google.com>
Auto-Submit: Stefano Duo <stefanoduo@google.com>
Commit-Queue: Stefano Duo <stefanoduo@google.com>
Cr-Commit-Position: refs/heads/main@{#1509199}