Support Encryption at Rest with a proper key vault

Follow-up of: https://github.com/canonical/microceph/issues/191

Although the limitation was clarified 2.5 years ago, MicroCeph should now be the first class citizen as the successor of the previous "Charmed Ceph". We need to support proper Encryption at Rest instead of saving the encryption key on disk. Charmed Ceph, in the meantime, is capable to save encryption keys at Hashicorp Vault instances.

https://canonical-microceph.readthedocs-hosted.com/latest/explanation/security/about-fde/#limitations
> Also note that the encryption key will be stored on the Ceph monitors as part of the Ceph key/value store.