Add support for fallback keys

CMakeLists.txt

include/mtx/requests.hpp

include/mtx/responses/sync.hpp

include/mtxclient/crypto/client.hpp

lib/crypto/client.cpp

lib/structs/requests.cpp

lib/structs/responses/sync.cpp

meson.build

subprojects/Olm.wrap

tests/e2ee.cpp

tests/requests.cpp