Join add and update schedule modals

What does this MR do?

Displays basic schedule

Joins add and edit schedule modals into a single component as they are very much alike

🇺🇦 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 and edit it (pencil icon)

Screenshots (strongly suggested)

No visual and functional changes are expected

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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 #292985 (closed)