✨(mailboxes) add mail provisioning api integration

We want people to create new mailboxes in La Régie.
This commit adds integration with intermediary dimail-api,
which will in turn send our email creation request to Open-Xchange.

src/helm/env.d/preprod/values.desk.yaml.gotmpl

@@ -84,6 +84,7 @@ backend:
       secretKeyRef:
         name: redis.redis.libre.sh
         key: url
+    MAIL_PROVISIONING_API_URL: "https://main.dev.ox.numerique.gouv.fr"
 
   createsuperuser:
     command: