rook-ceph-cluster: Support Prometheus rules customization

[gtirloni]

Is this a bug report or feature request?

• Feature Request

What should the feature do:
The rook-ceph-cluster chart could support creating extra Prometheus Rules allowing customization with better configuration locality (by avoiding the need to implement it in a separate chart or using some other mechanism). Additionally, if the rules from the imported mixin could be overriden, that would be helpful as well.

What is use case behind this feature:
Adding more rules for Ceph clusters or customizing aspects of the rules imported from mixins (e.g. severity).

Environment:
Prometheus deployed by prometheus-operator (from kube-prometheus-stack)

[travisn]

Have you tried using a tool such as Kustomize as suggested in the customize alerts topic? Customizing alerts has had a lot of discussion in the past, and further customization is just difficult to consider.