DAL/Node: improve the interface when requesting cells of the skip list

This MR does two things:

• Explicitly return [None] if the cell associated to the given skip list hash is not found in the sqlite backend. This translates to the use of opt_register on the RPC registration side, which returns 404 to the client if the value is [None], instead of crashing with 5xx code and a Caqti error.

• Add a function find_by_slot_id_opt in skip list's sqlite backend. The internal SQL request needs a join on the cell's hash to retrieve the cell content of the skip list whose slot id is given. Once the cell is obtained, its content could be inspected (e.g. to get the commitment hash) using the DAL plugin. This is needed when fetching slots from remote HTTP servers to validate their content against the commitment of published slots on L1.