[RFE] Support alternative datastores when deploying K3s clusters

[DillonN]

Is your feature request related to a problem? Please describe.
Currently when deploying a K3s cluster via a node provisioner such as Harvester, etcd is a required component. There is no way to deploy a cluster that uses an alternative datastore such as MySQL or PostgreSQL

Describe the solution you'd like
At a minimum it should be possible to deploy a cluster without any etcd nodes when using K3s. For a full experience, it would be best to have UI options on K3s clusters to select a datastore without having to manually set command line arguments.

Describe alternatives you've considered
It's probably possible to deploy a node pool with control and etcd both enabled, then just swap the datastore after creation. However this seems wasteful as etcd would still be running.

And of course it's possible to create the cluster outside of Rancher and then import it afterward. But this is losing some of the value add that Rancher has for deployment.

Additional context
Not sure if there'd be a way to work this into the etcd snapshot feature, or if that would be incompatible with clusters created this way.

K3s article on datastores: https://docs.k3s.io/datastore

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[DillonN]

Still relevant to me