Add oncall schedule helpers

What does this MR do?

Adds new helpers to the shift utils and associated specs. These are not used yet, but will be used in later MRs (see below).

Merge train

This MR is part of the on-call schedule merge train. It is one step in an overall effort to allow more detailed shifts to be displayed accurately. Full details in this issue: #324608 (closed)

1. Introduce new helper for shift utils <--- You are here!
2. Update shift width query
3. Consolidate Week and Day shift items
4. Render grid as single timeline cell and update day indicator
5. Remove old code & add integration spec
6. Update on-call test suite

All changes are consolidated in a sandbox MR here: !57300 (closed)

Screenshots (strongly suggested)

No visual change.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because there is no visual change.
• [n/a] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] 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
• [n/a] 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:

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

issue: #324608 (closed)