Send a message event to a room.
https://matrix.org/docs/spec/client_server/r0.5.0#put-matrix-client-r0-rooms-roomid-send-eventtype-txnid
t() :: %Polyjuice.Client.Endpoint.PutRoomsSend{ event_type: String.t(), message: map(), room: String.t(), txn_id: String.t() }