Add a user to a map
POST
{{cartes_api_base_url}}/maps/{{cartes_map_uuid}}/users
curl -X POST 'cartes_api_base_url/maps/048eebe4-8dac-46e2-a947-50b6b8062fec/users?username=xxx&can_create_markers=true' -F '{"name"=>"username", "value"=>"xxx", "datatype"=>"string"}=' -F '{"name"=>"can_create_markers", "value"=>"true", "datatype"=>"boolean"}='