Alpha/DAL: new attestation_lags parameter

This MR introduces a new parameter attestation_lags, which is a list of "lags" (offsets wrt to levels of published slots) at which attesters can attest for the availability of shard assigned to them. This dynamic approach should improve the DAL's latency.

The MR does not do more that this.

Currently the list is just [attestation_lag]. This is just an intermediary step, as it will allow to test the feature in a first step, without changing the existing tests too much.