Autoscaling for runners

[jjgriff93]

Currently we need to set a static number of instances. It would be desirable to set an autoscale rule for the scale set that triggers based on pending jobs from the GitHub API using the queued jobs web hook: https://docs.github.com/en/actions/hosting-your-own-runners/autoscaling-with-self-hosted-runners

Built-in autoscaling rules for scale sets are limited so this will require some investigation into whether this is possible, whether we'd need an intermediary functions etc. that calls ARM to bump up/down the instances etc.