feat(rook): add script to import external Ceph cluster credentials

[GreatSymphonia]

Summary

• Adds common/rook/import-external-cluster.sh, a one-shot script to import the shared Proxmox Ceph external cluster credentials into a target cluster's rook-ceph-external namespace
• Wraps the standard Rook external-cluster exporter flow (SSH to a Proxmox/Ceph node, run the exporter, apply the output) so it doesn't need to be redone manually per cluster

Test plan

• Used manually against production-v2 / k8s-shared during recent Ceph/CephFS setup work; no automated tests for this ops script