DAL: define a richer attestation status and propagate the info until dal_slot_repr

Stacked on !15320 (merged)

In this MR,

• we define a richer type for attestation status of slots (not just a flag, but also attestation ratio)
• we propagate the information until inside dal_slot_repr where the cells of the skip list are constructed (but not plugged yet)
• we adapt the different tests and functions to the new interface (we don't just take attested slot headers, but all published headers with their attestation statuses)