List conversation recipients endpoint

Per this answer the only way to retrieve all of the recipients of a conversation is to iterate through all messages in the conversation and manually pull out - and de-duplicate - the recipients from each message. This is both unnecessarily complicated and may involve many API calls which may incur additional costs.

The Core API should provide an endpoint to retrieve the list of all recipients in a conversation, mimicking the list of recipients that is populated when a new message or draft is created using the Front UI.