feat(dossier): prefill region champ (#8442)

* make regions champ prefillable

* add possible and example value

* add external_id and value validation

spec/controllers/api/v2/graphql_controller_spec.rb

@@ -9,6 +9,8 @@ describe API::V2::GraphqlController do
   let(:dossiers) { [dossier] }
   let(:instructeur) { create(:instructeur, followed_dossiers: dossiers) }
 
+  let(:memory_store) { ActiveSupport::Cache.lookup_store(:memory_store) }
+
   def compute_checksum_in_chunks(io)
     Digest::MD5.new.tap do |checksum|
       while (chunk = io.read(5.megabytes))
@@ -37,6 +39,9 @@ describe API::V2::GraphqlController do
   end
 
   before do
+    allow(Rails).to receive(:cache).and_return(memory_store)
+    Rails.cache.clear
+
     allow(APIGeoService).to receive(:departement_name).with('01').and_return('Ain')
     instructeur.assign_to_procedure(procedure)
   end
@@ -392,7 +397,7 @@ describe API::V2::GraphqlController do
         dossier
       end
 
-      context "for individual" do
+      context "for individual", vcr: { cassette_name: 'api_geo_regions' } do
         let(:procedure) { create(:procedure, :published, :for_individual, :with_service, :with_all_champs, :with_all_annotations, administrateurs: [admin]) }
         let(:query) do
           "{