Add to_device SSSS gossiping

include/mtx/events/collections.hpp

include/mtx/events/encrypted.hpp

include/mtx/events/event_type.hpp

lib/http/client.cpp

lib/structs/events.cpp

lib/structs/events/collections.cpp

lib/structs/events/encrypted.cpp

lib/structs/responses/common.cpp

tests/e2ee.cpp