Add GraphQL mutation for adding oncall rotation

What does this MR do?

This adds the ability to create On-Call rotations via the GraphQL API.

feature flag: Like all On-call MVC work, this is behind flag oncall_schedules_mvc.

GraphQL Mutation example:

Mutation:

mutation newRotation($input: OncallRotationCreateInput!) {
  oncallRotationCreate(input: $input) {
    oncallRotation {
      id
      name
      startsAt
      length
      lengthUnit
      participants {
        nodes {
          user {
            id
            username
          }
          colorWeight
          colorPalette
        }
      }
    }
    errors
  }
}

Query variables:

{
  "input": {
    "projectPath": "root/autodevops-deploy-test",
    "scheduleIid": "1",
    "name": "Rotation 242",
    "startsAt": {
      "date": "2020-09-19",
      "time": "09:00"
    },
    "rotationLength": {
      "length": 1,
      "unit": "WEEKS"
    },
    "participants": [
      {
        "username": "root",
        "colorWeight": "WEIGHT_500",
        "colorPalette": "BLUE"
      }
    ]
  }
}

Response:

{
  "data": {
    "oncallRotationCreate": {
      "oncallRotation": {
        "id": "gid://gitlab/IncidentManagement::OncallRotation/2",
        "name": "Rotation 242",
        "startsAt": "2020-09-18T21:00:00Z",
        "length": 1,
        "lengthUnit": "WEEKS",
        "participants": {
          "nodes": [
            {
              "user": {
                "id": "gid://gitlab/User/1",
                "username": "root"
              },
              "colorWeight": "500",
              "colorPalette": "blue"
            }
          ]
        }
      },
      "errors": []
    }
  }
}

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 #262857 (closed)