[Feature Request] Add existing k3s cluster

[goolzerg]

Describe the solution you'd like
Feature which allows adding already existing k3s clusters which were created manually

[orangedeng]

Can you provide more details about the importing existing k3s clusters use case? It's not quite the major purpose of auto3s.

Managing the cluster lifecycle from beginning is reasonable as all of the resources are created by autok3s. But it is not easy to import the cluster from intermediate state.