Use hidden friends to reduce overload sets

include/mtx/common.hpp

include/mtx/errors.hpp

include/mtx/events.hpp

include/mtx/events/account_data/direct.hpp

include/mtx/events/account_data/fully_read.hpp

include/mtx/events/aliases.hpp

include/mtx/events/avatar.hpp

include/mtx/events/canonical_alias.hpp

include/mtx/events/collections.hpp

include/mtx/events/common.hpp

include/mtx/events/create.hpp

include/mtx/events/encrypted.hpp

include/mtx/events/encryption.hpp

include/mtx/events/ephemeral/receipt.hpp

include/mtx/events/ephemeral/typing.hpp

include/mtx/events/guest_access.hpp

include/mtx/events/history_visibility.hpp

include/mtx/events/join_rules.hpp

include/mtx/events/member.hpp

include/mtx/events/messages/audio.hpp