Oncall-Rotations: Persist shifts job

What does this MR do?

This adds jobs to persist On-call Shifts into the DB.

To do this we have 2 background jobs which run:

Job	Description
IncidentManagement::OncallRotations::PersistAllRotationsShiftsJob'	Job to fan out other jobs for each rotation (Cron job. This is ran every 5 minutes.)
IncidentManagement::OncallRotations:: PersistShiftsJob	Job to calculate the shifts to be persisted, and persist them.

Since now we can persist shifts, we also change the ReadService to read persisted shifts for dates in the past, which we combine with the generated shifts in the future.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #262860 (closed)