Is your feature request related to a problem? Please describe.
I want to use btrfs reflinks/snapshots to take backups and save disk space further, and want it to only make backups after the world is completely saved.

Describe the solution you'd like
Add a custom backup type that executes custom command, with world folder and backup destination folder exported as environment variables, to use a external program to make backups.
So user can use a command like cp --reflink=always $WORLD_DIR $BACKUP_DEST to make reflink backups with this mod.

Describe alternatives you've considered
Use cron to run backup jobs, but may create a broken backup when the world is partially saved

Additional context
No need to implement restoration for custom backups. The backups isn't managed by the mod.

@HeatherComputer smells like another potential application of SPI's... 👀

For reference: SimpleBackups rejected shell functionality because of security concerns, see ChaoticTrials/SimpleBackups#25 (comment).