Implement Pool Health Proposal

[friegger]

Summary

To determine the availability and health of each machine pool, we propose introducing heartbeats and status conditions of pools.

Similar to Kubernetes' node heartbeats & status conditions, there are 2 parties involved in this:

• The pool itself, regularly reporting its status by patching the *Pool resource's status and acquiring / keeping a lease object within the ironcore-machinepool-lease namespace. For references on leases, see the leases Kubernetes documentation.
• A pool controller within the IronCore control plane that updates a pool object's status conditions depending on the heartbeats of the pool.

It was decided to only implement this mechanism for MachinePools.

ToDos

• heartbeats
  • machinepoollet creates and updates Lease in ironcore-machinepool-lease namespace
• controllers ironcore/controller/*pool
  • machinepool-lifecycle-controller that monitors the Lease and Ready Condition
• kustomize config adjustments

Motivation

See the proposal's motivation section