Enable GitHub workflows for PRs with the ok-to-test label

[ivanvc]

Allow to run GitHub workflows for Pull Requests with the ok-to-test label using the prow bot command /ok-to-test. This was already done in the etcd repo (refer to the issue etcd-io/etcd#16956 and its PR etcd-io/etcd#16965).

[yashkundu]

I want to do it, @jmhbnz , can you please assign this to me?

[jmhbnz]

Certainly! Please reach out to @ivanvc or I if you have any questions 🙏🏻

/assign @yashkundu

[yashkundu]

I have implemented it on #117.