move config validation to cli package instead of in specific store implementations

[BK]

#111 still does config checking within the store. (Specifically, checking for the metadata extension config to exist)

If a config is required for cavorite to run, we should be checking it and bailing (or set a default) in the code that is reading the config. This should live here: https://github.com/discentem/cavorite/blob/main/internal/cli/init.go#L72

The config code should own the config validation, if that makes sense.