executors: Allow non site-admins to add executors

Created by: eseliger

Executors attached to sub queues (#28166) should be possible to add by non site-admin users. They aren't able to touch anything the user wouldn't be able to, so it's safe to allow users to deploy those whenever they like. Users might even choose to temporarily just start up an executor on their own machine, to get something worked through quickly. (sg run executor essentially). Therefore, we need to have secrets per-user. Maybe we can just reuse personal access tokens and give them a special scope.