Rollup node: generalize and move Snapshot utilities in lib_layer2_store

Temporarily takes ownership : @antoine.lanco

What

This MR moves the snapshot utility functions in the library lib_layer2_store.

Fixes #7403 (closed).

Why

These functions need to be reused by the evm node to implement snapshots.

How

The snapshot utility functions are parameterized (functor) by the header type.

Depends on !14181 (merged).