Add support for MSC2545

CMakeLists.txt

docs/dirs.dox

include/mtx/events/collections.hpp

include/mtx/events/event_type.hpp

include/mtx/events/mscs/image_packs.hpp

include/mtxclient/http/client.hpp

lib/http/client.cpp

lib/structs/events.cpp

lib/structs/events/collections.cpp

lib/structs/events/mscs/image_packs.cpp

lib/structs/responses/common.cpp

tests/events.cpp