feat(demarche): create and prefill a dossier with POST request (#8233)

* add base controller for public api

* add dossiers controller with basic checks

* create the dossier

* ensure content-type is json

* prefill dossier with given values

* mark a dossier as prefilled

When a dossier is prefilled, it's allowed not to have a user.

Plus, we add a secure token to the dossier, which we will need later to set a
user after sign in / sign up.

* set user as owner of an orphan prefilled dossier

When a visitor comes from the dossier_url answered by the public api,
the dossier is orphan:
- when the user is already authenticated: they become the owner
- when the user is not authenticated: they can sign in / sign up / france_connect
and then they become the owner

So here is the procedure:
- allow to sign in / sign up / france connect when user is unauthenticated
- set dossier ownership when the dossier is orphan
- check dossier ownership when the dossier is not
- redirect to brouillon path when user is signed in and owner

* mark the dossier as prefilled when it's prefilled
(even with a GET request, because it will be useful later on, for
exmample in order to cleanup the unused prefilled dossiers)

* system spec: prefilling dossier with post request

app/models/concerns/dossier_prefillable_concern.rb

@@ -3,10 +3,11 @@
 module DossierPrefillableConcern
   extend ActiveSupport::Concern
 
-  def prefill!(champs_attributes)
-    return if champs_attributes.empty?
+  def prefill!(champs_public_attributes)
+    attr = { prefilled: true }
+    attr[:champs_public_attributes] = champs_public_attributes.map { |h| h.merge(prefilled: true) } if champs_public_attributes.any?
 
-    assign_attributes(champs_attributes: champs_attributes.map { |h| h.merge(prefilled: true) })
+    assign_attributes(attr)
     save(validate: false)
   end
 end