refactor client, add json library

- move data_struct to ClientDataPackage.hpp
- move Message definition to Message.hpp
- move Message code to Message.cpp
- move global time variable to its own namespace called esptime
  - time accessible via esptime::rtc
- Message now uses ClientDataPackage instead of data_struct
- ram_caching now uses ClientDataPackage instead of data_struct
- switched from pointer definition of Message to default constructor
- move headers to hpp files where applicable
- added ArduinoJson library (does not increase space by a lot)