The difference between `KeyList` and `ThresholdKey` is ambiguous and poorly documented

At some point, the decision was made to combine `KeyList` and `ThresholdKey` objects, such that if a `threshold` value is set, then a `ThresholdKey` protobuf is produced, otherwise producing a `KeyList` protobuf

This invites misuse: a `KeyList` is intended to have specialized behavior related to files. It isn't just a `ThresholdKey` where N of N keys are required to sign.

1. If a developer *wants* the special `KeyList` behavior, but sets a `threshold` value without understanding the implications, they might not know that suddenly they no longer have a `KeyList` at all
2. If a developer doesn't know anything about the specialized file related behavior of a `KeyList`, and assumes setting no threshold == requiring N of N signatures (logical, IMO), they could get be blindsided by specialized `KeyList` behavior