Make DAL attestations shorter when only the first slots are used

What

Instead of using number_of_slots bits for each lag for which the baker attests at least one slot, we truncate the attestation of a lag in chunks, with 1 bit to state if one is reading the last chunk related to the current lag and 7 (here, parametrizable) bits to state the attestation of the next 7 slots.

Why

With the raise of the number of slots to 160 and the dynamic lag, the size of the DAL attestation would grow too much.

Manually testing the MR

CI should be green. The best commit to understand the representation is Proto/Alpha: Update DAL attestation representation mli file with an example which introduces a detailed example.