RunnerSet is basically ReplicaSet for Runners.
It is responsible for maintaining number of runners to match the desired one. That is, it creates missing runners from `.Spec.Template` and deletes redundant runners.
Similar to ReplicaSet, this does not support rolling update of runners on its own. We might want to later add `RunnerDeployment` for that. But that's another story.
Yusuke Kuoka