Invalidate Pages API cache with Projects::ProjectTransferedEvent

What does this MR do and why?

For GET /api/internal/pages?host=HOST. This is the API endpoint that gets called by GitLab pages when a request comes in and we haven't cached in gitlab-pages what needs to be served for this.

Given that the satisfiedThreshold for writing headers for a response is 0.5s, ideally, this would be even faster than the threshold set for web-pages. We think 0.5s for serving a static page is an acceptable target.

Since this information generally does not change a lot, we could add a cache for the JSON response of this endpoint to help the metrics.

The cache introduced in #363867 (closed) (behind a feature flag), needs to be invalidated in some application events (&7920 (closed)).

This MR

This MR invalidates this cache with the Projects::ProjectTransferedEvent introduced in !92620 (merged).

Related to: #364064 (closed)

FAQ

• Does it make sense to guard these changes under a FF?
  • The main feature, the cache itself, is behind a FF (cache_pages_domain_api, introduced in !88956 (merged))

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