Add support for Matrix Room V12 additional creators parameter. #816
Description
As you might be aware, Matrix Room v12 specification created a "creator" role (which element refers to as owner) which is set at room creation and cannot be modified in any way with all permissions and infinity power level.
The specs for version 12 state that creator is determined via the sender of the m.room.create event plus anyone designated by the additional_creators within the m.room.create.
I can find use for being able to designate several co-creators for a room. Outside of making a room with a friend who has equal share of power. If I am making a room and I have a couple backup alts on other home servers I want to ensure have the same power as I do, I can make that happen. As well as give my personal copy of draupnir all the power it needs for rooms I create for it to moderate.