create a migration story for new versions of type "Config" and "State"

[aybabtme]

Both the Config and State types have a Version int field in them so that when decoding a config or state object, I can know if they were encoded first with an older version of the config.

This is a stop-gap solution to make it merely possible to handle a breaking change in the layout of these types, but it doesn't provide a solution for doing breaking changes to these types and writing migrations/remedies for upgrading old Config/State values to new versions.

So we need to make a mechanism to (for both State and Config, using only Config as an example here):

• make it possible to decode Config values from any previous version
• migrate a Config version v-1 to a version v (à la go fix), prompting for user input if necessary (see the huh package)
• create a pipeline that can take any Config at version v-n and bring it to version v (prompting for user input if necessary)
• this should all be testable and tested
• it should be easy to write new migrations for breaking changes