Add micro benchmark for ledger state snapshots

Snapshots of the current Cardano mainnet ledger are big (2-3 GB). Taking a snapshot is an expensive operation that has been identified as a performance bottleneck before (see https://github.com/IntersectMBO/ouroboros-consensus/issues/868).

To discover performance regressions introduced by the changes to the snapshot logic (e.g. https://github.com/IntersectMBO/ouroboros-consensus/pull/1319), we need a micro benchmark analogous to [`mempool-bench`](https://github.com/IntersectMBO/ouroboros-consensus/tree/68b9799eebdc7037a99668762ddc69c7e78c106c/ouroboros-consensus/bench/mempool-bench).