Add endpoint for getting a role's users

[robines]

Role is basically just a group of permissions. It's not directly tied to users. To tie a role to a user, one must use either UserOrgRole, UserGangRole, or UserGangSectionRole. This will give the role to the user on a specific hierarchical level (on a specific object instance of an Organization, Gang or Section).

This PR adds an endpoint which gets all UserOrgRole/UserGangRole/UserGangSectionRoles and their users, based on the specified role id. The object is also fetched, meaning we can easily it in frontend.

Response looks something like:

[
  {
    "user": UserDto,
    // org_role when role is given on Org level, gang_role for Gang, and section_role for Section
    "gang_role": {
      "created_at": "2024-10-29T23:37:39.950095Z",
      "created_by": UserDto, // Who gave the role
      // organization when org_role, gang when gang_role, section when section_role
      "gang": {
        "id": 4,
        "name_nb": "Markedsføringsgjengen",
        "name_en": "Markedsføringsgjengen",
        "abbreviation": "MG",
        ...
      }
    }
  },
  ...
]