While it's possible to share the content between different event types
in the usual case some event types have slightly different contents if
they are sent out as a to-device event vs a room message event.
The canonical example for this are key verification events where the
to-device version has a transaction id field but the room message
version uses event relationships for the same purpose.
This patch makes it possible for to-device events to have different
content structs. Type aliases are used where a common struct can be
used.
