SCORU: interpret internal L1 to L2 messages

Context

Fixes #3199 (closed).

Manually testing the MR

dune exec tezt/tests/main.exe -- -f sc_rollup.ml -t 'Alpha: node advances PVM state with internal messages' -v

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Provide automatic testing (see the testing guide).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR