Add rotation assignee to the schedule view

What does this MR do?

A small MVC for:

• #262860 (closed)
• #262865 (closed)

Where we add the rotation assignee component and the supporting popover for showing the participant start and end date. This merge adds no logic for position calculation as we will break this effort into multiple merges as the logic is complex.

Merge description	You are here
Add rotation assignee component	🤘
Calculate position on Grid dynamically	cell
Add support for day, week and month presets	cell

🇺🇦 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

Screenshots (strongly suggested)

Designs

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 #262860 (closed)