Feature request: snapshot retention

I'm setting up dragonfly instance with a following snapshot config:

```yaml
spec:
# ...
snapshot:
cron: "*/2 * * * *"
persistentVolumeClaimSpec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 20Gi
```

It works beautifully, however, when left unattended, instance can rack up thousands of backup files within a month.

What would be great is if operator cleaned up older backups, something like:
```
spec:
# ...
snapshot:
cron: "*/2 * * * *"
persistentVolumeClaimSpec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 20Gi
retention: "1d" # only keep up to 1 day worth of backups
```