Add Edit Oncall Rotation Service

What does this MR do?

As part of #262862, this adds an Edit Rotation service which allows On-call Rotations, and their participants (IncidentManagement::Oncallparticipant) to be updated.

• We allow addition, removal and modification of participants, via an upsert
• Removal of participants results in soft-delete, where we set the is_removed attribute to true
• We allow modification of the rotation object through regular Rails #update.

Follow-up:

• Code to handle re-calculating the Oncall Rotation during an edit
• GraphQL mutation to expose this service

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 #262862