Add serializeNones config to allow omitting None fields in case classes

[lolgab]

Motivation

When enabling serializeDefaults together with optionsAsNulls this writes optional fields that are None as null. in JSON null is not the same as a missing field. This new setting changes upickle behavior to serialize None as a missing field instead of null.

TODOs

• Add check to disallow serializeNones = false together with optionsAsNulls = false because it doesn't make sense to use these two settings together.