Check self-hosted runners are online
# What does this PR do?

#18905 checks if the docker could be launched inside the runners.

However, the runners could be offline due to some unknown reasons, and we are not aware of this problem (job hangs forever) so far.

This PR adds a check for runner being online or offline.

However, it might happen that a runner becomes offline in the middle of a workflow run. This situation is not easy to deal with, and we still need to prevent such situation. Therefore, a new scheduled (per hour) workflow is created to check runner availability.


_The documentation is not available anymore as the PR was closed or merged._