WIP: Extend WebHooks to support HTTP Negotiate/SPNEGO (Kerberos)

What does this MR do?

Extend WebHooks to support HTTP Negotiate/SPNEGO (Kerberos) -- fixes #31078 (moved).

This required replacing HTTParty with HTTPI.

This is a WIP MR because I haven't yet tested it against an actual Kerberized webserver. But I want to post it so folks know that this is in-progress.

Why was this MR needed?

We need Kerberos authentication so that our webhook receiver can tell that a webhook call in fact came from Gitlab (and thus that the information therein is trustworthy). Because the webhook is installed on user-owned repositories, there is no way to use a shared secret for authentication.

Does this MR meet the acceptance criteria?

• [/] Changelog entry added, if necessary
• [/] Documentation created/updated
• [/] API support added
• Tests
  • [/] Added for this feature/bug
  • [/] All builds are passing
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#31078 (moved)