SORU: better Arith PVM integration with DAL

Context

Fixes #3995 (closed)

This MR is needed to be able to run DAL tests with SCORU (PVM Arith) that can be dynamically parameterised by DAL parameters.

Here is the documentation of the added directive in PVM Arith (that can be found in sc_rollup_arith.mli)

   - an input [dal:<e>:<num_p>:<s1>:<s2>:...:<sn>] is interpreted as a directive
   to provide the DAL parameters to the PVM, where:
     - <e> is the attestation lag;
     - <num_p> is the number of pages;
     - each <si> is a slot to which the PVM subscribes to.

    DAL parameters can be set at most once. At each Start_of_level of some level
    [L] inbox message and if DAL is enabled for the rollup (via the directive
    above), the PVM will request the pages of the slots it is subscribed to. The
    (attested) slots that are actually fetched at level [L] are those published
    at level [L - e - 1]. Note that providing some DAC data via a DAL page will
    prevent from fetching the subsequent DAL pages.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR