Support cluster-api MachineDeployments

Once we have CRDs (#6849), we should be able to support MachineDeployments for Nodes with kops. This should enable e.g. better interaction with the cluster autoscaler, or (limited) dynamic manipulation of nodes without reinvoking the kops CLI.

cluster-api is still alpha, so we won't make this the default (nor even required for the foreseeable future).

This feature is purely for tackling non-master Nodes; master nodes are much more complex; they also might not be as useful to manage through cluster-api (in my opinion).

Start with the CRDs from #6849 and the existing kops handling for non-master Nodes. Define how MachineDeployments should interact with the cluster autoscaler and dynamic node changes without making cluster-api required or the default. Done means non-master Nodes can be managed through MachineDeployments while master-node management remains out of scope.

Written by the indexing model from the issue text.