support querying for older snapshots

[zhekson1]

Why

Currently, aggregator endpoints seem to only return recent cardano-db snapshots. This is an issue if, for whatever reason, we need to bootstrap from older snapshots (i.e. older node versions).

What

Support querying for snapshot digests by time of creation and/or epoch number. Ideally incorporated in the mithril explorer website, but at least via the aggregator API.