MSC3383: Include destination in X-Matrix Auth Header (#3383)
* add proposal for adding the dest to federation auth headers
Signed-off-by: Jan Christian Grünhage <jcgruenhage@famedly.com>
* address compatibility concern regarding fed auth headers
The MSC already addressed old implementations receiving events from
newer implementations sending the additional field, but not the other
way around, which is added in here
* clarify the problem MSC3383 aims to solve
* clarify which verification msc3383 talks about
* clarify what happens with unknown destinations in msc3383