Move autoscale into core server rather than needing a dedicated runner

Description including problem, use cases, benefits, and/or goals

Being able to autoscale GitLab runners is awesome, but it's a bit annoying to need to manually configure a meta-runner, let alone the waste of compute. Let's move that functionality into GitLab server itself.

Proposal

Links / references

Related to gitlab-ce#14812, but more specific.