Delete older backups

[woolfg]

The sidecar can produce a lot of backups which grow fast. Especially, when uploading backups somewhere (e.g. by using a script or woolfg/mysql-backup-sidecar-gs) , it should be possible to define a pattern when old backups get deleted. Maybe just by specifiying a max age in days which would then delete all full backups older than the defined value.