Add OncallShifts read service and specs

What does this MR do?

Code originally written by @syasonik, and extracted from !50006 (merged).

• This adds a service which, given a time frame, can return shifts for a given rotation.
• This is currently limited to return generated shifts, but will be extended to return persisted shifts too in a later MR.

#262860 (closed) is comprised of several pieces:

• Add shifts table (!49423 (merged))
• Add ability to generate shifts from rotation (!49665 (merged))
• Add graphql support for querying shifts (!50006 (merged))
• Add shifts read service (!50135 (merged) - this MR)
• Persisting historical shifts (TBD)

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)