Use std::{variant,optional} instead of boost

CMakeLists.txt

examples/crypto_bot.cpp

examples/room_feed.cpp

examples/simple_bot.cpp

include/mtx/events/collections.hpp

include/mtx/events/common.hpp

include/mtx/events/create.hpp

include/mtx/events/messages/audio.hpp

include/mtx/events/messages/file.hpp

include/mtx/events/messages/image.hpp

include/mtx/events/messages/video.hpp

include/mtx/responses/common.hpp

include/mtx/responses/login.hpp

include/mtxclient/http/client.hpp

lib/structs/responses/common.cpp

lib/structs/responses/sync.cpp

tests/client_api.cpp

tests/e2ee.cpp

tests/events.cpp

tests/responses.cpp