optionalUnion
fun Arguments.optionalUnion(displayName: Key, description: Key, typeName: Key? = null, shouldThrow: Boolean = false, vararg converters: GenericConverter, validator: <Error class: unknown class><Any?> = null): OptionalCoalescingConverter<Any>
Create an optional union converter, for combining other converters into a single argument - with the caveat of type erasure.
This function will automatically remove converters if they were previously registered, so you can use pass it the results of the usual extension functions.