updated HelloWorldPlugin for Hivemq-spi v2.0.0 (refs #2509)
added ClientService example (refs #2509)
added advanced callbacks that leverage the newly introduced services and also some new simple callbacks with improvements
changed group Id of SPI and version number
changed behavior of SendListOfAllClientsOnPublish callback to send out a client list only when topic is fetch/all/clients
Changed bridge type to manual
corrected cron syntax of LogBrokerUptime callback