Add SnapshotItem to storage-plus

[ethanfrey]

Let's build the equivalent type to SnapshotMap, but for Item. We can query this at any snapshotted height. Can use the same logic for selected checkpoints (+1 for code reuse/refactor)

In particular, I want this, so I can query a snapshot of total_weight at the beginning of the block the proposal was created on . This is a vulnerability mentioned in one of the cw3-flex-multisig snapshoting prs/issues (can't find it now)

You can use this for snapshotting total weight in cw4-* and exposing history to cw3 callers in the same PR or in a separate.