GraphQL mutation for creating Escalation Policies

What does this MR do?

Adds a mutation for creating escalation polcies.

Mutation GraphQL Query / Response

Request w/ input:

mutation CreateEscalationPolicy($input: EscalationPolicyCreateInput!) {
  escalationPolicyCreate(input: $input) {
    escalationPolicy {
      id
      name
      description
      rules {
        status
        elapsedTimeSeconds
        oncallSchedule {
          name
        }
      }
    }
    errors
  }
}


{
  "input": {
    "projectPath": "root/autodevops-deploy-test",
    "name": "Test Escalation Policy 3",
    "description":"Test description",
    "rules": [
      {
        "oncallScheduleIid": "12",
        "elapsedTimeSeconds": 60,
        "status": "ACKNOWLEDGED"
      }

    ]
  }

Response:

{
  "data": {
    "escalationPolicyCreate": {
      "escalationPolicy": {
        "id": "gid://gitlab/IncidentManagement::EscalationPolicy/5",
        "name": "Test Escalation Policy 3",
        "description": "Test description",
        "rules": [
          {
            "status": "ACKNOWLEDGED",
            "elapsedTimeSeconds": 60,
            "oncallSchedule": {
              "name": "schedule"
            }
          }
        ]
      },
      "errors": []
    }
  }
}

Background

This is part of the Escalation Policies MVC, the technical planning of which can be found here.

To achieve this, this work is being broken up into multiple MRs:

• Add the Escalation policy/rule tables | !60685 (merged)
• Backfilling the Escalation policies/rules for existing Oncall Schedules | !62233 (merged)
• Create GraphQl types for Escalation policy/rules | !61439 (merged)
• Create the Escalation policies via the UI | Frontend MR | Backend MR - THIS MR
• Add an Escalation model which represents an Escalation, and the services needed for business logic around that (Escalating, sending emails etc) | !62361 (closed)
• Background workers / Cron jobs to actually perform the business logic | TODO

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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.

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 #268356 (closed)