DAL: remember recent skip list cells in the L1 context

Following the PoC done within Milestone %Adaptive DAL POC, this is the first MR towards integrating Adaptive DAL.

Why this MR?

In Adaptive DAL, we need another data source to determine whether a slot is attested to some threshold defined by a given smart rollup (instead of looking at the DAL bitset in L1 blocks' metadata). We could either choose the hard way. That is: redoing all the computations starting from (DAL) attestations posted by bakers, retrieving the number of shards they are assigned, and redoing the maths, ... But we could also just remember the successive skip list cells' in the L1 context at every level (without explicitly indexing by level to avoid exploding the size of the context).

In addition to our use case, this method makes the indexation of the cells of the skip list from the DAL node much simpler.