Add Carte champs support to the API

spec/serializers/champs/carte_champ_serializer_spec.rb

@@ -0,0 +1,18 @@
+describe Champs::CarteChampSerializer do
+  describe '#attributes' do
+    subject { Champs::CarteChampSerializer.new(champ).serializable_hash }
+
+    context 'when type champ is carte' do
+      let(:geo_area) { create(:geo_area) }
+      let(:champ) { create(:type_de_champ_carte).champ.create(geo_areas: [geo_area]) }
+
+      it {
+        expect(subject[:geo_areas].first).to include(
+          source: GeoArea.sources.fetch(:cadastre),
+          numero: '42',
+          feuille: 'A11'
+        )
+      }
+    end
+  end
+end