feat(demarche): sign up when prefilling a dossier (#8247)

* store prefill params in session

Instead of using query params on /dossier/new, we assume the user comes
from /commencer/:path, which is the new prefill link.

There, we store the prefill params in session, and use them to prefill
the dossier when creating it, in /dossiers/new.

* spec: cover the case

* review: serialize with json instead of yaml

* review: rename method

* review: store only query params

* review: comment why we dont override already stored params

app/controllers/users/dossiers_controller.rb

@@ -1,6 +1,7 @@
 module Users
   class DossiersController < UserController
     include DossierHelper
+    include QueryParamsStoreConcern
 
     layout 'procedure_context', only: [:identite, :update_identite, :siret, :update_siret]
 
@@ -304,7 +305,7 @@ module Users
       )
       dossier.build_default_individual
       dossier.save!
-      dossier.prefill!(PrefillParams.new(dossier, params).to_a)
+      dossier.prefill!(PrefillParams.new(dossier, retrieve_and_delete_stored_query_params).to_a)
 
       if dossier.procedure.for_individual
         redirect_to identite_dossier_path(dossier)