New Reaper deployment mode: Control Plane

[adejanovski]

We want to allow deploying a single instance of Reaper which will repair all K8ssandraClusters managed by a single control plane.
This requires to create a new deployment mode in Reaper (aside from PER_DC and SINGLE), which we can name "CONTROL_PLANE".
A Reaper custom resource would need to be created separately prior to the K8ssandraCluster objects creation, with this mode.
Any K8ssandraCluster object without Reaper settings would be registered by the operator into the control plane Reaper and repairs would be managed by it.

We would disallow enabling Reaper in clusters if the control plane instance exists to keep things simple in the first iteration.
This means either you have one centralized Reaper managing all clusters or you have individual Reaper instances per cluster.

Definition of Done

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1. the Reaper API has a new deployment mode named CONTROL_PLANE
2. A Reaper running with memory storage, the depoyment mode must be CONTROL_PLANE or SINGLE
3. A CONTROL_PLANE reaper comes up without contacting any K8ssandra clusters (for storage or registration)
4. A SINGLE reaper won't deploy more than once
5. The k8ssandraCluster object has a field in the reaper spec that defines which Reaper to register with
6. In the K8ssandraCluster spec, a reaperRef is added to point to the Reaper instance that will handle the repairs
7. When a CP Reaper instance exists, k8ssandra-operator registers all K8ssandraClusters in it
8. If a K8ssandraCluster object gets created with Reaper settings while a control plane Reaper exists, the K8ssandraCluster objects should override control plane Reaper with their own defined Reaper settings.