Add the edit modal for rotations and abstract add/edit form

What does this MR do?

A small MVC for #262862 where we add the UI for the edit modal and abstract the add/edit form to be a shared container set. This merge contains the UI only, the actual backend code will be hooked up once the backend of #262862 is ready.

🇺🇦 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	🤘
Add GraphQL mutation for edit modal when backend is ready	TBA

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 #262862