Add graphql type for oncall shifts

What does this MR do?

Related issue: #262860 (closed).

#262860 (closed) is comprised of several pieces:

• Add shifts table (!49423 (merged))
• Add ability to generate shifts from rotation (!49665 (merged))
• Add graphql support for querying shifts (!50006 (merged))
• Persisting historical shifts (TBD)

This MR adds support for querying IncidentManagement::OncallShifts via GraphQL.

Ultimately, shifts will be comprised of persisted past OncallShifts & unsaved future OncallShifts. This MR is limited to only unsaved shifts for the entire time range, but this will allow the frontend to integrate with the API in parallel with completing the backend logic. Note that this content is still feature flagged, so a changelog is not included.

Sample query:

Noteworthy: Querying shifts requires two arguments - startsAt and endsAt.

query schedules {
   project(fullPath: "root/autodevops") {
    incidentManagementOncallSchedules {
      nodes {
        iid
        name
        description
        timezone
        rotations {
          nodes {
            shifts(startsAt: "2020-12-05 22:26:12-00:00", endsAt: "2020-12-14 22:26:12-00:00") {
              nodes {
                participant {
                  id
                  colorWeight
                  colorPalette
                  user {
                    username
                  }
                }
                endsAt
                startsAt
              }
            }
          }
        }
      }
    }
  } 
}

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 #262860 (closed)