DAL/Node: get_slot_commitment RPC fetches commitment from skip-list

What

This MR updates the GET /levels/<published_level>/slots/<slot_index>/commitment to fetch the commitment directly from the corresponding cell in the skip-list store.

The MR then enhances the skip-list migration test:

• by calling the cells_of_level RPC and checking the answer
• by calling the new RPC and checking the answer

Why

It will allow us to test that the skip-list store has the right contents.

Manually testing the MR

CI should suffice.