Add SkippableOptionFields value for deserialization only

[cmeeren]

I am consuming a poorly documented/consistent upstream API. I want to deserialize to option even if the field is omitted, without having to wrap everything in Skippable with all the extra work this entails. I can't see that this is currently possible. If I use SkippableOptionFields.Always, None fields are skipped also on serialization, but I want to do this only for deserialization.