Proto: expand attestations_aggregate committee type with DAL contents for each slot

What

In the Attestations_aggregate operation, change the type of the committee to (Slot.t * dal_content option) list

However, the encoding ignores DAL contents and decodes all of them to None, so the semantics is still the one of aggregate without DAL for now. This will be updated in future MRs. That's why tests and encodings are not affected yet.

Why

So that we can start implementing aggregate-with-DAL logic in upcoming MRs.

How

Add two functions tmp_to_old_committee and tmp_of_old_committee to satisfy typing while clearly marking places where the logic needs to be updated in the future.

Follow-up issue: #7935 (closed)