Silent Mode: Block Gitlab::HTTP outbound requests

Problem

From gitlab-org/geo-team/discussions#5059 (comment 1379241537):

Since the right place to suppress a number of features is probably in Gitlab::HTTP, I'm thinking that it may be most efficient if groupgeo implements that linchpin upfront. Then the downstream groups don't need to coordinate around an unmerged branch. They could just toggle Silent Mode, test their features, and make any further adjustments (fix errors, unblock critical requests, block missed requests, add tests, improve UI/UX, etc).

Shall we add an issue to track us doing that as a priority 1? I think it is a weight 2, if my findings in !96415 (closed) are accurate.

Proposal

• Implement blocking of Gitlab::HTTP when Silent Mode is enabled.
• Make sure that CI is passing.
• Document that Silent Mode breaks a lot of things by blocking many outbound HTTP requests.

Important note: It is acceptable and expected when enabling Silent Mode for it to cause many breakages since further work is scheduled to make everything work nicely. Silent Mode is still an Experiment.