RKE2Config support for MachinePools

[anmazzotti]

Describe the solution you'd like:
Currently the bootstrap controller expects a RKE2Config owned by a Machine.
However, when referenced by a MachinePool, the owner will be the MachinePool itself.

For example:

apiVersion: cluster.x-k8s.io/v1beta1
kind: MachinePool
metadata:
  name: capi-machinepool-mp-0
  namespace: default
spec:
  clusterName: capi-machinepool
  replicas: 3
  template:
    spec:
      bootstrap:
        configRef:
          apiVersion: bootstrap.cluster.x-k8s.io/v1beta1
          kind: RKE2Config
          name: capi-machinepool-mp-0

apiVersion: bootstrap.cluster.x-k8s.io/v1beta1
kind: RKE2Config
metadata:
  labels:
    cluster.x-k8s.io/cluster-name: capi-machinepool
  name: capi-machinepool-mp-0
  namespace: default
  ownerReferences:
    - apiVersion: cluster.x-k8s.io/v1beta1
      blockOwnerDeletion: true
      controller: true
      kind: MachinePool
      name: capi-machinepool-mp-0
      uid: 0d99ccb9-80d6-4842-b1de-0a6f9b05d7c5
spec:
  agentConfig:
    additionalUserData: {}
    format: cloud-config
  privateRegistriesConfig: {}

Also be aware that the kubeadm implementation does take special actions when the config is owner by a MachinePool.
Possibly this applies to RKE2 as well.

Why do you want this feature:

RKE2 support for MachinePools

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]