Extend PVM interface with proofs about outbox

Context

To call an L1 contract from the L2, a rollup can push a message in its outbox at some L. After the commitment at level L is finalized, anyone can trigger the execution of the L1 contract call provided a proof that this message is indeed part of the outbox.

This MR extends the PVM to allow the construction of these proofs.

Fixes: #2927 (closed)

Manually testing the MR

dune exec src/proto_alpha/lib_protocol/test/unit/main.exe -- test "^\[Unit\] sc rollup arith$"

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