Add GraphQL mutation to promote an issue to an epic

What does this MR do?

Exposes promote issue to epic through GraphQL mutation

Promote to epic within project parent group

• GraphqQL mutation

mutation {
  promoteToEpic(input: {
    projectPath: "gitlab-org/gitlab-shell", 
    iid: "1"     
  }) {    
    issue {
      id
      iid
      title
      state
      milestone {
        id
        title
      }
    }
    epic {
      id
      iid
      title
    }
    errors
  }
}

• Response with success

{
  "data": {
    "promoteToEpic": {
      "issue": {
        "id": "gid://gitlab/Issue/2",
        "iid": "1",
        "title": "First issue",
        "state": "closed",
        "milestone": {
          "id": "gid://gitlab/Milestone/9",
          "title": "v3.0"
        }
      },
      "epic": {
        "id": "gid://gitlab/Epic/5",
        "iid": "3",
        "title": "First issue"
      },
      "errors": []
    }
  }
}

• Response with error

{
  "data": {
    "promoteToEpic": {
      "issue": {
        "id": "gid://gitlab/Issue/2",
        "iid": "1",
        "title": "First issue",
        "state": "closed",
        "milestone": {
          "id": "gid://gitlab/Milestone/9",
          "title": "v3.0"
        }
      },
      "epic": null,
      "errors": [
        "Issue already promoted to epic."
      ]
    }
  }
}

Promote to epic to a specified group

• Graphql mutation

mutation {
  promoteToEpic(input: {
    projectPath: "gitlab-org/gitlab-shell", 
    iid: "1",
    epicGroupPath: "gitlab-org/gitlab-sugroup-1"
  }) {    
    issue {
      id
      iid
      title
      state
      milestone {
        id
        title
      }
    }
    epic {
      id
      iid
      title
    }
    errors
  }
}

• response with success

{
  "data": {
    "promoteToEpic": {
      "issue": {
        "id": "gid://gitlab/Issue/2",
        "iid": "1",
        "title": "First issue",
        "state": "closed",
        "milestone": {
          "id": "gid://gitlab/Milestone/9",
          "title": "v3.0"
        }
      },
      "epic": {
        "id": "gid://gitlab/Epic/6",
        "iid": "1",
        "title": "First issue"
      },
      "errors": []
    }
  }
}

• response with error

{
  "data": {
    "promoteToEpic": {
      "issue": {
        "id": "gid://gitlab/Issue/2",
        "iid": "1",
        "title": "First issue",
        "state": "closed",
        "milestone": {
          "id": "gid://gitlab/Milestone/9",
          "title": "v3.0"
        }
      },
      "epic": null,
      "errors": [
        "Issue already promoted to epic."
      ]
    }
  }
}

Promote to epic to an inexistent group

• Graphql mutation

mutation {
  promoteToEpic(input: {
    projectPath: "gitlab-org/gitlab-shell", 
    iid: "1",
    epicGroupPath: "gitlab-org/gitlab-sugroup-1/abracadabra"
  }) {    
    issue {
      id
      iid
      title
      state
      milestone {
        id
        title
      }
    }
    epic {
      id
      iid
      title
    }
    errors
  }
}

• response

{
  "data": {
    "promoteToEpic": null
  },
  "errors": [
    {
      "message": "The resource that you are attempting to access does not exist or you don't have permission to perform this action",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "promoteToEpic"
      ]
    }
  ]
}

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 #233974 (closed)