Allow the add rotation modal to create a new rotation

What does this MR do?

A small MR for the remaining part of #262857 (closed) where we link up the actual backend code for the mutation to create a rotation and update the associated specs. The next step of this merge is to extend the spec coverage via the Apollo mock, however that is left until the frontend of the rotation grid is hooked up from: !49680 (merged)

🇺🇦 It is behind the feature flag oncall_schedules_mvc and the minimal license is GitLab Premium 👀 To check Enable feature flag=>Navigate to Operations/On-call schedule => Click Add schedule on empty state or direct link is your_project/-/oncall_schedules e.g. http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/oncall_schedules. After a first schedule is created you'll be able to see it

Merge	You are
Add base mutation	🤘
Extend spec coverage for mutation	!50268 (merged)

Screenshots (strongly suggested)

N/A

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 #262857 (closed)