Add a champs/dossier_link endpoint and use it to fetch dossier info

app/controllers/champs/dossier_link_controller.rb

@@ -0,0 +1,11 @@
+class Champs::DossierLinkController < ApplicationController
+  before_action :authenticate_logged_user!
+
+  def show
+    if params[:dossier].key?(:champs_attributes)
+      @dossier_id = params[:dossier][:champs_attributes][params[:position]][:value]
+    else
+      @dossier_id = params[:dossier][:champs_private_attributes][params[:position]][:value]
+    end
+  end
+end