SCORU/Node: new storage layout with L2 blocks

Context

This MR removes many independent tables in irmin by reorganizing the storage layout of the rollup node around the notion of layer 2 blocks.

This MR is a first step in moving the store from irmin to indexed files and to fix the storage consumption issues.

Fixes #4390 (closed).

Depends on !7101 (merged).

Manually testing the MR

dune exec tezt/tests/main.exe -- -f sc_rollup.ml rpc api

TODO

• Backport to mumbai rollup node