Add support for policy rules

CMakeLists.txt

include/mtx/events/collections.hpp

include/mtx/events/event_type.hpp

include/mtx/events/policy_rules.hpp

include/mtxclient/http/client.hpp

lib/http/client.cpp

lib/structs/events.cpp

lib/structs/events/collections.cpp

lib/structs/events/policy_rules.cpp

lib/structs/responses/common.cpp

meson.build

tests/events.cpp