Add Futureproof<T> wrapper type, use for GuardDisabled enum

The Futureproof type lets you serialize and deserialize types whose representations might change (most useful for enums that might grow additional variants). It uses #[serde(untagged)] to accomplish this.

This gets used in order to make the disabled field of Guard more robust against future guard disablement reasons being added.

A test was also added to verify correct behaviour of the new type.