add includeScheduledContent flag to content API

when a user has scheduled content they want to preview on a site or do some testing on, currently the only way to fetch the content from the content API is to manually include the date as a userAttribute:

const targetDate = new Date();
targetDate.setDate(targetDate.getDate() + 10); // example of a date 10 days from now

builder.get( .., {
userAttributes: {
date: targetDate
}
})

But what if you dont know the date of a given content? It would be great to have a flag for includeScheduledContent, similar to includeUnpublished