Adaptive DAL POC: phase 2. Extend PVM Sig with a new reveal request

In this MR, we add a new reveal request case to reveal type:

  | Request_adal_page of {
      page_id : Dal_slot_repr.Page.t;
      attestation_threshold_per_mil : int;
    }

to become

type reveal =
  | Reveal_raw_data of Sc_rollup_reveal_hash.t
  | Reveal_metadata
  | Request_dal_page of Dal_slot_repr.Page.t
  | Request_adal_page of {
      page_id : Dal_slot_repr.Page.t;
      attestation_threshold_per_mil : int;
    }
  | Reveal_dal_parameters
      (** Request DAL parameters that were used for the slots published at
          the current inbox level. *)

For the answers, we'll still use, for the moment, the case:

  | Dal_page of Dal_slot_repr.Page.content option

of type

type reveal_data =
  | Raw_data of string
  | Metadata of Sc_rollup_metadata_repr.t
  | Dal_page of Dal_slot_repr.Page.content option
  | Dal_parameters of Sc_rollup_dal_parameters_repr.t