DAL: new skip list cells ordering compatible with dynamic lag

This MR introduces a new compare function for the cells of the DAL skip list that is both compatible with the current/old static/legacy ordering and with the upcoming dynamic lag ordering.

This ordering is used in the next function to ensure that the cells are inserted following this new ordering and in the search function, alonside a new lag argument, to find target cells.

To keep the same interface when producing proofs, the search function is wrapped in another helper that looks for target cells using all possible lags.

This MR is not sufficient to support dynamic lag. The functions page_id_is_valid and import_level_is_valid need be adapted

• Currently, those function are static. Given the (current) attestation lag, they can say if an import is valid or not
• In the future, with dynamic lag, The check cannot be static anymore, as we don't know a-priori with which lag a slot would be attested.