Issue #13398 - Improve LoginAuthenticators to handle Proxy-Authenticate

[znight1020]

This pull request resolves issue #13398 by enhancing server-side authenticators to support proxy authentication.

Summary of Changes

1. Authenticator Enhancements:

   • Server-side authenticators like BasicAuthenticator, DigestAuthenticator and SPNEGOAuthenticator no longer handle only hardcoded WWW-Authenticate headers.
   • A new setProxyMode(boolean) method has been added to the base LoginAuthenticator in both jetty-security and jetty-ee9-security.
   • When proxy mode is enabled, these authenticators now dynamically issue 407 challenges with Proxy-Authenticate headers and process credentials from the Proxy-Authorization header, using new internal helper methods.

2. Test Refactoring:

   • The parent HttpClientAuthenticationTest has been refactored to use a new protected boolean isProxyMode() method (which defaults to false), allowing it to test both authentication modes.
   • A new HttpClientProxyAuthenticationTest subclass is introduced, which simply overrides isProxyMode() to return true, reusing the entire parent test suite for comprehensive validation of the new proxy mode.

Resolves #13398

[sbordet]

@znight1020 excellent work!
Please follow the few renamings that I suggested, and then we should be good to merge.

[znight1020]

Thanks for the feedback, @sbordet!

I've implemented the requested renamings and Javadoc improvements, but before I push the changes, I had a quick question regarding testInfiniteAuthentication, which I've detailed in the unresolved conversation above.

[sbordet]

@znight1020 I don't see the changes after review.
Did you push them?

[znight1020]

Sorry for that, @sbordet. I've just pushed the requested changes now!

To get the tests passing, I've temporarily updated testInfiniteAuthentication() to use startBasic(), which resolves the NullPointerException. However, my original question from the thread still stands: #13452 (comment)

Could you please take a look at the reasoning in that thread when you have a moment?

[sbordet]

Just a typo in the javadocs, and fix testInfiniteAuthentication() and we should be good.

[znight1020]

I've corrected the typo in the Javadocs and updated testInfiniteAuthentication.
Thanks again for all the feedback and guidance through the process!

[sbordet]

@znight1020 excellent work!
This PR is approved, we'll wait on the CI build and then merge it.
Thanks!