Backup Management: Retention

User story:
As a PMM user, I need to be able to specify the retention period in a number of artifacts to store,  when I'm scheduling a backup so it will be removed from the storage after some time.

UI/UX:
see Figma

Acceptance criteria

• the User is able to specify the retention on UI as a number of artifacts to be stored 
• the User able to specify the retention in API/CLI
• the backup files are deleted after their number exceeded retention  (past retention)
• the User is able to see the number of retention on the Backup Inventory view
• Scheduling documentation is updated with the new option 

Out of scope:

• "Weekly or daily retention". Just a number of backup artifacts to store 

Suggested implementation:

• could borrow some code from https://github.com/xrutayisire/react-js-cron

How to test:
TBD

Details:
TBD