feat(sync): Let the clients set layers UUID

[almet]

This make it possible to synchronize datalayers before their creation on the server, allowing at the same time to solve issues related to them not being saved (e.g. duplication of geometries)

This does the following:

• Introduce a new datalayer._future_uuid property, used before the server actually saves the datalayer ;
• Add this information to the metadata used for syncing ;
• Change the default route for datalayer creation to contain an UUID ;
• Use this UUID as the source of truth when saving the datalayer on the server.