Invalidate gitlab-pages internal api cache with 'Pages domain updated' event

Context

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), needs to be invalidated in some application events (&7920 (closed)).

What is this issue

This issue is introduce the cache invalidation for the Pages domain updated event.

To invalidate the cache the event must contain at least:

• the project id
• project namespace id
• project root namespace id