Skip merge process if an existing PR exists and has the "hold" label

[simonpasquier]

Sometimes, we have to wait before synchronizing our downstream (for instance, there's a global merge freeze). There's no way to tell the bot to skip the update except disabling the GitHub action which is not very user-friendly because someone has to remember to enable it back.
It would be useful for the bot to check if a PR already exist and if it has the "hold" label (signalling that it needs to wait), it wouldn't try to update the PR.

[simonpasquier]

Example: openshift/prometheus-operator#235 (comment)