Provide flexible backup strategy. Charmed PostgreSQL 14 provides only manual AWS S3 and Ceph Rados GW backups.

Hi!

It is not a bug, but a feature request. The charm allows only backups to AWS S3 (or any other S3 compatible) or to Ceph Rados GW. It seems rather narrow and would be great to allow some of the:
* local backups on the disk [to enable backup management with Charmed Duplicity](https://charmhub.io/duplicity), or just store backups on NFS volume.
* FTP/SFTP/rsync

The current list of integrations for backups doesn't seem compatible with most On-Premises deployments. Even Azure doesn't have native support for S3 protocol. It limits the flexibility of charmed PostgreSQL 14 compared to previous charmed versions.

Even after the backup medium is configured, it is not clear how to configure Juju-managed scheduled backups as the charm doesn't provide any schedule, only a juju-action that should be triggered manually.