Querying across versions for migrations

[colinparsonscom]

Originally brought up by @demsey2 in issue #188. Would be super useful to have some sort of functionality to complete migrations across versions!

@tywalch has mentioned the difficulty of determining an API for this, which is true. Open to any ideas or suggestions with how to achieve this using ElectroDB's current functionality, because my company will need to do a data migration from a previous version soon!

Although this is achievable by (1) using DynamoDB's API to query and update the returned items from production with the new values that ElectroDB will compute, then (2) pushing out the updated schema using ElectroDB like normal, ideally there'd be some "ElectroDB safe / approved" manner of doing this. It doesn't even have to live with the schema it's modifying; it'd just be nice to have some place in the code where we have a bit of ElectroDB-style type safety while structuring this migration.