Plugin Support

[ZBX-22532] add zbxflag package

* commit '89aa037313b94e505e311dae4fd0cce3bc27498f':
  [ZBX-22532] update usage message
  [ZBX-22532] add zbxflag package unit tests
  [ZBX-22532] add protocol version to help message
  [ZBX-22532] fix plugin common flag handler help message
  [ZBX-22532] fixed changelog entry
  [ZBX-22532] added common flag handling function for plugins
  [ZBX-22532] added changelog entry
  [ZBX-22532] add zbxflag package