Add support for KEDA ScaledObjects as an alternative to regular HPAs

What does this MR do?

Add support for KEDA ScaledObjects as an alternative to regular HPAs, see https://keda.sh/docs/2.10/concepts/scaling-deployments/

This will enable us to use custom metrics from Prometheus, Stackdriver or any other metric source for autoscaling any GitLab workload, in particular Sidekiq.

KEDA ScaledObjects are disabled by default. When enabled the HPAs are automatically disabled (ScaledObjects create then own HPAs) and the ScaledObjects configuration defaults to the HPA configuration when applicable and can be fully overridden with the keda.* values.

Related issues

• https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/17127+
• https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15720+

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.