Allow creation of custom scalars from polymorphic types

[Melodeiro]

Right now an exception is thrown with lists of polymorphic types before fun willGenerateGraphQLType(type: KType): GraphQLType? since #801. My suggestion is to allow users to handle this situation with custom Coercing implementation and throw if no custom scalar type were provided. It is possible already but only with a hack:

override fun willResolveInputMonad(type: KType): KType {
    return when { // MessageContent is sealed class, but MessageContentInput is data class and actually does nothing
        type == typeOf<List<MessageContent>>() -> typeOf<List<MessageContentInput>>()
        else -> super.willResolveInputMonad(type)
    }
}

override fun willGenerateGraphQLType(type: KType): GraphQLType? = when (type.classifier) {
    MessageContentInput::class -> MessageContentScalar // uses Coercing<MessageContent, String>
    else -> null
}