Implement RemoteMachine pooling

[jnummelin]

To be able to use RemoteMachines as part of scalable control planes, we need to implement some sort of pooling for them. CAPI control plane objects always refer to XYZMachineTemplates and as such those are kinda impossible to implement for RemoteMachines.

So instead we could implement some sort of pooling for the RemoteMachines.

We could create a new kind for a pooled machine, say PooledMachine:

kind: PooledRemoteMachine
pool: mypool
spec:
  address: 1.2.3.4
  sshKey: mykey
  ...

Then in the RemoteMachineTemplate we could refer to a pool, something like:

kind: RemoteMachineTemplate
spec:
  pool: mypool

When various CAPI controllers would create a RemoteMachine out from a RemoteMachineTemplate we could have an mutating webhook to:

• select one un-used PooledRemoteMachine, we need to track the usage status in status fields
• copy over the address etc. details to the created RemoteMachine