GroupInfo as a "public" struct

[kkohbrok]

I will note that this promotes GroupInfo into a "public" / "on-the-wire" object, as opposed to always being wrapped in a Welcome. In the internal join case, the GroupInfo is always sent encrypted to the new joiner; in the external join case, it isn't necessarily encrypted (up to the application). Doesn't need to be a blocker, but might be worth elaborating on in an implementation document.

Originally posted by @bifurcation in mlswg/mls-protocol#529 (review)