Preflight check to validate folder for ETCD machines exist

**What would you like to be added**:
vSphere Folder checks are already there for control plane and worker nodes. Add the same logic for ETCD machines.

CAPV logs:
```
E0906 17:24:06.570332 1 controller.go:317] controller/vspherevm "msg"="Reconciler error" "error"="failed to reconcile VM: folder '/SDDC-Datacenter/vm/capv/abhnvp' not found" "name"="abhnvp-etcd-ndrx9" "namespace"="eksa-system" "reconciler group"="infrastructure.cluster.x-k8s.io" "reconciler kind"="VSphereVM"
```

EKS Anywhere version: v0.11.1

**Why is this needed**:
Preflight improvements

Locate the existing vSphere folder checks for control-plane and worker nodes, then trace the CAPV reconciliation path for ETCD machines. Add equivalent preflight validation for ETCD folders and confirm that a missing folder is reported before reconciliation, matching the behavior shown in the CAPV log.

Written by the indexing model from the issue text.