Expose is_auditor user role via API [RUN AS-IF-FOSS]

What does this MR do?

#300127 (closed)

Similar to how we expose is_admin when using the user API as an admin, provide is_auditor boolean attribute.

As an admin:

% curl --header "PRIVATE-TOKEN: $TOKEN" "http://172.16.123.1:3000/api/v4/users/103" | json_pp
{
   ...
   ...
   "is_admin" : false,
   "is_auditor" : true,
   ...
   "name" : "audit or",
   ...
   "username" : "auditor",
   ...
}

As a non-admin:

{
  "id": 103,
  "username": "auditor",
  "name": "audit or",
  ...
  ...
  # does NOT have "is_auditor" field
 
}

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