New message_type to record phone call via API

Currently, there is no method for 3rd party services like Aircall, DialPad, TalkDesk, etc to record a conversation for a call activity that was not triggered from within Chat. They each adopt slightly different hacks to log calls, e.g., record as a closed ticket, or record as a closed chat conversation. There should be a dedicated message_type in the API in order that calls can be distinguished from other conversations.