[Feature Request] Delete unmanaged snapshots after X days

[thomas315]

Is your feature request related to a problem? Please describe

Snapshots that were created with ISM operation "snapshot" can't be automatically deleted after a defined retention period.
Using Snapshot Management is not an option because we need to create the Snapshots via ISM to then be able to use convert_index_to_remote operation to restore the Snapshot as "Searchable Snapshot".

Describe the solution you'd like

Being able to set a retention to delete Snapshots that were created with ISM.
Different ways I can think of:

• Adding an delete_after parameter to the ISM Snapshot operation
• Adding a delete_backed_snapshot: true parameter to the delete ISM operation, that is immediately deleting the backed snapshot
• Adding a different kind of Snapshot Policy (Snapshot Retention Policy?) that is not matching against indices with an index pattern, but is matching against existing Snapshots (without a policy attached). With this option it doesn't matter how the Snapshot was created by it could always be deleted after configured retention period within this Snapshot Policy.

Related component

Storage:Snapshots

Describe alternatives you've considered

Of course this can be managed externally with any script or by Curator but it would be nice to have this feature built in.

Additional context

No response