Ways to convey the Ratchet Tree in Messaging Layer Security

The Messaging Layer Security (MLS) protocol needs to share its ratchet_tree object to welcome new clients into a group and in external joins. While the protocol only defines a mechanism for sharing the entire tree, most implementations use various optimizations to avoid sending this structure repeatedly in large groups. This document describes a way to convey these improvements in a standardized way and to convey the parts of a GroupInfo object that are not visible to an intermediary server.