Allow whitelisting some keys for idempotency check #79

Recently we have had the try-runtime tool fail in tanssi because of a state mismatch when running the idempotent check. The cause was 2 keys in pallet system: events and eventsCount, which changed because one of our migrations emitted an event to signal the start of the migration. Since events don't really affect chain state, we would like to ignore them.

Not sure how to implement this, we could hardcode some known keys and ignore them in the state diff comparison, or simply allow users to pass some flag with a list of keys to ignore.