Add gitlab-pages admin ping rake task

What does this MR do?

This change adds plumbing for the new gitlab-pages gRPC admin API. All this can do at the moment is to ping gitlab-pages from a rake task, but it does exercise authentication and (when using TCP) encryption.

None of this is useful to users yet, I'm setting up this part first to get all the integration work (including Omnibus) out of the way.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• 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
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

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