Add oncall_users to oncall schedule Graphql type

What does this MR do?

This adds oncall_users as a field on the OncallScheduleType, in order to expose who is oncall for the particular Schedule at the given time.

GraphQL queries and responses

Query

{
  project(fullPath: "root/autodevops-deploy-test") {
    incidentManagementOncallSchedules {
      nodes {
        oncallUsers {
          username
        }
      }
    }
  }
}

Example Response

{
  "data": {
    "project": {
      "incidentManagementOncallSchedules": {
        "nodes": [
          {
            "oncallUsers": [
              {
                "username": "namespace1"
              },
              {
                "username": "namespace10"
              }
            ]
          },
          {
            "oncallUsers": [
              {
                "username": "rozanne_shields"
              }
            ]
          }
        ]
      }
    }
  }
}

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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 #251140 (closed)