SCORU/Node: Accuser mode

Context

A smart rollup run in "accuser mode" follows the layer 1 chain and computes commitments but does not publish them. Only when a conflicting commitment (published by another staker) is detected will the "accuser node" publish a commitment and participate in the subsequent refutation game.

Fixes #3450 (closed).

Notes:

• the accuser node publishes a commitment for the level where there is another commitment branch even if there is another staker who also agrees with its commitment, to ensure the accuser node will participate in a refutation game.

Manually testing the MR

dune exec tezt/tests/main.exe -- -f sc_rollup.ml refutation accuser

TODO

• Backport to Mumbai/016 rollup node