MicroCeph path traversal issue in the remote-import API

Canonical MicroCeph versions from the squid and tentacle track are vulnerable to a path traversal issue in the remote-import API. Holders of a trusted cluster mTLS certificate (such as enrolled cluster members) or join token can manipulate files in an imported remote cluster within the /var/snap/microceph confinement. This would allow daemon disruption and pollution of the cluster state.