[scd] Automate expired subscription eviction

[barroco]

#1116 introduced a CLI command to evict expired subscriptions following the discussions in #1074.
Multiple options have been mentioned during scoping of the feature in order to periodically delete expired subscriptions:

Approach	Execution notes	Pros	Cons
1. Recommend users to use a cron job running the eviction job	Update deployment code to support it with configuration flags	Datastore independent
2. Set TTL in migration scripts	Implementation: #1134	Garbage collection managed by CRDB	No TTL feature in Yugabyte (YSQL)
3. Recommend users to set the TTL	Add instructions in the README with an example on how to proceed		We want to encourage standardized deployment and limit customisations
4. Add a cron job inside core-service like rid	Implement inside core-service the new cron job

This issue should be used to decide on the preferred approach(es).
Please comment this issue and I will update the description to reflect the progress.