Gitea instance-level runners

[bachorp]

It would be great to allow for instance-level Gitea runners.

[gabriel-samfira]

Instance level runners are on the TODO list, but I need to stabilize what we already added before adding more. Initially I thought about reusing the Enterprise concept from GitHub and equating that to "instance" level runners in Gitea, but it doesn't really fit. A Github Endpoint may have multiple "enterprises" while a gitea endpoint would be the "enterprise".

So I need to add a whole new entity type, which is not really hard, but it is a lot of code to add right now. It will be added, but most likely after we manage to actually stabilize enough to have a v0.2.0 release of GARM.