Backup Management: Scheduling of backups

User story:
As a PMM user, I need to be able to schedule a backup so it will be executed at a specific time.

UI/UX:
see Figma

Acceptance criteria

• User able to specify the Service/cluster to make a backup (same as for on-demand)
• User able to specify the type of backup (same as for on-demand)
• User able to specify on the UI time or day of the week and interval between backups
• Backup Documentation updated with a new feature

Out of scope:

• The retention period for the scheduled backup
• User able to specify by API/CLI the time for scheduled backups in "crontab" format
• Inventing our own "scheduler"

Suggested implementation:
Implement scheduler inside pmm-managed: probably https://github.com/go-co-op/gocron / https://github.com/robfig/cron

Store scheduled jobs inside postgres:
 - frequency as cron expression
 - start at
 - last run

• retry attempts
• ...
   TBD: store backup schedule in own table or inside generic schedule table (parameters can be as JSONB)

How to test:
TBD

Details:
TBD