Job Router

What does this MR do?

This MR implements the first iteration of the Runner Job Router. Runner discovers the kas URL and polls it for jobs (instead of GitLab directly).

This MR depends on Implement Job Router discovery endpoint for run... (gitlab!214239 - merged) as it needs that endpoint to get the kas URL.

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?

Relates to Implement Runner job proxying (gitlab-org/cluster-integration/gitlab-agent#771).