Add Gitaly N+1 detector tests

Adds Gitaly N+1 detector tests to be run on a schedule against the testbed.

I've added PREPROD_HOST_URL to the CI/CD settings because it could be useful for other pipelines.

This depends on a couple of changes:

• gitlab-qa needs to pass the CLI flag that enables the Gitaly N+1 detector: gitlab-org/gitlab-qa!231 (merged)
• we need an admin account for the preprod env: https://gitlab.com/gitlab-org/gitlab-ce/issues/57911#note_165375918

And currently we have to run the tests as an admin user otherwise we can't toggle the feature flag. I've opened an issue to fix this: https://gitlab.com/gitlab-org/gitlab-ce/issues/61230

Pipeline schedules have been set up:

• Preprod: https://gitlab.com/gitlab-org/quality/performance/pipeline_schedules/24676/edit (Every Sunday at 1 AM UTC - currently inactive)
• Nightly: https://gitlab.com/gitlab-org/quality/performance/pipeline_schedules/24679/edit (I was going to delete this after I finished testing but I decided to leave it because it doesn't use many resources and could be useful for tracking changes).

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58905