Join etcd nodes to K8s cluster

**What would you like to be added**: External unstacked etcd nodes added to K8s Nodes

**Why is this needed**: For example to add node monitoring with k8s entities to etcd nodes

The issue names no files, tests, or entry points. Start by reviewing how EKS Anywhere represents Kubernetes nodes and external etcd nodes, then clarify the expected node-monitoring behavior and acceptance criteria. Done should be a defined, testable way to represent external unstacked etcd nodes as Kubernetes entities.

Written by the indexing model from the issue text.