Support for podDisruptionBudget

We're using gitlab runner in K8s and we'd like have a more controlled runner manager terminations on node disruptions events, so the addition of a podDisruptionBudget config to the chart would be useful.

We're currently configuring it in a custom chart, but it'd be great if the gitlab-runner chart could provide support for it.

Note that there is an old (and now closed) MR for working this out !57 (closed)