[Core API] New endpoint for message seen receipts

We've released an update which makes message seen information available through our API.

You can find documentation on the new endpoint here.

In the case of channels which support the recording of seen-by information, the results of this endpoint will include each recipient who has seen the message (seen_by) as well as the timestamp of their first view (first_seen_at). For channels that do not support seen-by recording, there will be a single result with only a timestamp, which represents the very first time the message was seen by any recipient.