Add support for Redis Clustering (Cluster or Sentinel)

[EStork09]

What would you like to see!

The ability to set Redis to support clustering. Right now with USE_REDIS_CLUSTER = True I am getting a large amount of MOVED making it seem as though engine isn't handling the concept of being in a cluster well. I tried the headless service as well as just the regular service and it still resulted in that.

Product Area

Other

Anything else to add?

If redis clustering is already supported that is fine, it just might require some better documentation.

[github-actions]

The current version of Grafana OnCall, at the time this issue was opened, is v1.5.0. If your issue pertains to an older version of Grafana OnCall, please be sure to list it in the PR description. Thank you 😄!