Implement attributes filtering

[jmpesp]

SCIM allows for filtering the returned object's fields to only the attributes specified in the attributes query parameter, plus some mandatory ones. From https://datatracker.ietf.org/doc/html/rfc7644#section-3.4.2:

The example query below requests the userName for all Users:

   GET /Users?attributes=userName
   Host: example.com
   Accept: application/scim+json
   Authorization: Bearer h480djs93hd8

   The following is an example response to the query above:

   HTTP/1.1 200 OK
   Content-Type: application/scim+json

   {
     "schemas":["urn:ietf:params:scim:api:messages:2.0:ListResponse"],
     "totalResults":2,
     "Resources":[
       {
         "id":"2819c223-7f76-453a-919d-413861904646",
         "userName":"bjensen"
       },
       {
         "id":"c75ad752-64ae-4823-840d-ffa80929976c",
         "userName":"jsmith"
       }
     ]
   }

   Note that in the above example, "id" is returned because the "id"
   attribute has the "returned" characteristic of "always".