Add rke2 flavor support

[AshleyDumaine]

What type of PR is this?

/kind feature

What this PR does / why we need it:
This PR adds template support for rke2 as a flavor when generating a cluster. it does require that the rke2 bootstrap/controlplane providers are already installed and also that you specify a rke2-specific k8s version:

$ k --kubeconfig kubeconfig-rke2.yaml get no
NAME                            STATUS   ROLES                       AGE     VERSION
test-rke2-control-plane-27jpc   Ready    control-plane,etcd,master   3m3s    v1.29.1+rke2r1
test-rke2-control-plane-hrpq6   Ready    control-plane,etcd,master   5m48s   v1.29.1+rke2r1
test-rke2-control-plane-t2m8g   Ready    control-plane,etcd,master   44s     v1.29.1+rke2r1
test-rke2-md-0-hvxhz            Ready    <none>                      5m35s   v1.29.1+rke2r1
test-rke2-md-0-wds2d            Ready    <none>                      5m35s   v1.29.1+rke2r1
$ clusterctl describe cluster test-rke2
NAME                                                       READY  SEVERITY  REASON  SINCE  MESSAGE
Cluster/test-rke2                                          True                     103s
├─ClusterInfrastructure - LinodeCluster/test-rke2          True                     7m38s
├─ControlPlane - RKE2ControlPlane/test-rke2-control-plane  True                     103s
│ └─3 Machines...                                          True                     3m57s  See test-rke2-control-plane-7rkf4, test-rke2-control-plane-mg6cj, ...
└─Workers
 └─MachineDeployment/test-rke2-md-0                       True                     4m32s
   └─2 Machines...                                        True                     6m28s  See test-rke2-md-0-88q7h-hkgsc, test-rke2-md-0-88q7h-hvqcn

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #8

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[eljohnson92]

it looks like the rke2 provider info is built into clusterctl, so I don't think this is necessary