Add support for local instance backups

Make `gel instance backup` and `gel instance restore` work with local instances. Backups should be stored to some well-known location (`sharedir/backups`). Like with Cloud backups should be classified as `automated` and `manual`. Automated backups would be taken before certain destructive operations (e.g in-place upgrade, schema migrations etc).

The most straightforward approach would be to use [`pg_basebackup`](https://www.postgresql.org/docs/current/app-pgbasebackup.html) in combination with [`--incremental`](https://www.postgresql.org/docs/current/continuous-archiving.html#BACKUP-INCREMENTAL-BACKUP). We should use [`pg_combinebackup`](https://www.postgresql.org/docs/current/app-pgcombinebackup.html) to squash incremental backups to only keep `N` most recent automated backups.