SCORU: Interpret messages in L2 node

Context

Handle messages for the PVM inside the L2 node.

In order to unblock subsequent tasks, this MR implements a naive approach which does not concern itself with the boot sector or overflowing number of ticks or messages - it shoves all the messages it gets per level into the PVM and hopes for the best. See #2717 (closed), #2722 (closed) and #2727 (closed) for follow-ups.

Manually testing the MR

dune exec -- tezt/tests/main.exe \
  -f sc_rollup.ml \
  -t 'Alpha: node advances PVM state with messages' \
  -t 'Alpha: node boots into the initial state'

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