Add new GraphQL type for milestones

- added the milestone type to be displayed with investments
- the corresponding spec

spec/lib/graphql_spec.rb

@@ -678,6 +678,16 @@ describe "Consul Schema" do
     end
   end
 
+  describe "Milestone" do
+    it "formats publication date like in view" do
+      milestone = create(:milestone, publication_date: Time.zone.parse("2024-07-02 11:45:17"))
+
+      response = execute("{ milestone(id: #{milestone.id}) { id publication_date } }")
+      received_publication_date = dig(response, "data.milestone.publication_date")
+      expect(received_publication_date).to eq "2024-07-02"
+    end
+  end
+
   describe "Budget investment" do
     it "does not include hidden comments" do
       budget = create(:budget)