Allow for edit of oncall schedules

What does this MR do?

A tiny MVC as a start to add the frontend of supporting editing of on call schedules. This effort is broken into two merges because the code for the backend is yet to be made available to this branch. We also use this merge to create a generic component to handle the create and edit form.

Merge	You are
In ital GraphQL and frontend	🤘
Hook frontend up with working backend code and update specs	TBA

🇺🇦 This feature sits behind behind the feature flag oncall_schedules_mvc and the minimal license is GitLab Premium 👀

rails c
Feature.enable(:oncall_schedules_mvc)

• 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.

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 #262849