Remove some redundant includes and use string_views in some places

examples/online_backup_exporter.cpp

include/mtx/events.hpp

include/mtx/events/collections.hpp

include/mtx/events/encrypted.hpp

include/mtx/events/messages/location.hpp

include/mtx/events/mscs/image_packs.hpp

include/mtx/events/nheko_extensions/event_expiry.hpp

include/mtx/events/nheko_extensions/hidden_events.hpp

include/mtx/events/policy_rules.hpp

include/mtx/events/power_levels.hpp

include/mtx/events/presence.hpp

include/mtx/events/reaction.hpp

include/mtx/requests.hpp

include/mtx/responses/common.hpp

include/mtx/responses/crypto.hpp

include/mtx/responses/device.hpp

include/mtx/responses/empty.hpp

include/mtx/responses/sync.hpp

include/mtx/secret_storage.hpp

include/mtxclient/crypto/client.hpp