SCORU: Use bootsector in L2 node

Context

#2722 (closed)

Uses the boot sector when initializing the PVM state.

Manually testing the MR

dune exec -- tezt/tests/main.exe -f sc_rollup.ml -t 'Alpha: client gets boot sector via RPC'

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