Adds reference to httpx2-negotiate-sspi package migrated to httpx2.

Summary

Adds a 3rd party library built on httpx2.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.