Expose Conversation Seen information for teammates via the API

Would like the addition of an API endpoint for Conversation/Message opened.

• This datetime is briefly referenced in the Analytics Report for Handle Time.

This would be useful, in at least one way, for capacity analysis by measuring how much time a person spends gathering information for an email and crafting a reply, not just the time spent crafting the reply once that information is gathered. We understand the error cases inherent in this approach, but believe it would be more useful than not.