Add support for basic push rules

src/base/copy-helper.hpp

src/base/json-utils.hpp

src/client/CMakeLists.txt

src/client/push-rules-desc-p.hpp

src/client/push-rules-desc.cpp

src/client/push-rules-desc.hpp

src/tests/CMakeLists.txt

src/tests/base/json-utils-test.cpp

src/tests/client/push-rules-desc-test.cpp