add more list management features to newsletter portion of the API (specifically, retrieve a list of unsubbed users from a list)

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We'd like to maintain a list in the Newsletters app via the API that contains the active users on our site over the last 90 days. We plan to use this list to send maintenance notifications and similar items. We'd also like to respect people's subscription preferences - if they unsubscribe, we'd like our list to remember that.

It's my understanding that a given unsubscribe from a newsletter message only applies to one recipient list in Newsletters, and that these unsubscribes are not globally tracked. These two facts make it difficult to achieve what we would like to do - we could create a new list each time, with only the folks from the last 90 days, but if anyone has unsubbed before, it won't be reflected on the new list. Or, we could repeatedly remove everyone from a single list, then import the new batch, but it seems like this would also remove the unsub.

In chat with Kyle Kern, he suggested we could download the list of unsubbed folks, then import the updated list, and then import the list of unsubs on top of that, updating the preference for anyone that exists in both lists to be the proper unsubbed state. However, this would have to be done manually, as currently the Newsletter lists API doesn't support downloading a list of unsubbed addresses. It seems like adding this to the API would be the easiest solution to this rat's-nest, and then we could manage the multi step import process, which seems reasonable, due to the semi-weird needs of the thing we'd like to do.