Adds the initial version of RunnerDeployment that is intended to manage RunnerSets(#1), like Deployment manages ReplicaSets.
This is the initial version and therefore is bare bone. The only update strategy it supports is `Recreate`, which recreates the underlying RunnerSet when the runner template changes. I'd like to add `RollingUpdate` strategy once this is merged.
This depends on #1 so the diff contains that of #1, too. Please see only the latest commit for review.
Also see https://github.com/mumoshu/actions-runner-controller-ci/runs/471329823?check_suite_focus=true to confirm that `make tests` is passing after changes made in this commit.
Yusuke Kuoka