More straightforward customization of ReadWriters behaviour

[szymon-rd]

Right now, to make changes to the Option serialization (put null in json), or to use snake_case, one has to override corresponding methods in the AttributeTagged. I propose two ways to improve the DX in this process:

• Add traits to be used as add-ons (with cake pattern) in order to configure the serializers, for example traits such as SnakeCaseNaming or NoneValueAsNull:

object CustomPickle extends AttributeTagged, SnakeCaseNaming, NoneValueAsNull

• Rename AttributeTagged to be more descriptive of its purpose and easier to remember. Maybe even CustomizableUpickleApi or CustomNamingUpickleApi.