Allow specific restic snapshots to be marked for persistence

[jgilfoil]

Describe the feature you'd like to have.
I'd like to be able to mark a specific snapshot to be immune to the retain: settings that trigger the restic forget command on the backend. I'm thinking that as a first step for this feature, being able to specify a tag that is excluded from the forget command:

    retain:
      daily: 10
      within: 3d
      keep-tag: donotdelete

Then the restic forget command would be executed like:

restic forget --keep-daily 10 --keep-within 3d --keep-tag donotedelete

This would allow the user to go into a restic pod, and do

restic tag --add donotdelete 05e9f599

to tag a particular snapshot and exclude it from the forget command.

What is the value to the end user? (why is it a priority?)
In the event of a data loss incident, where recovery might not be possible immediately, but you still want to retain the data for later analysis or restoration past the retain date, being able to go in and tag a snapshot for persistence would be useful.

How will we know we have a good solution? (acceptance criteria)
User has a mechanism to retain a snapshot past the retain window settings.

Additional context
The only way I can think of to do this today is to delete the existing replicationsource to avoid the forget cleanup policy from being applied.