Send MQTT message when cache warming event takes place

[mountaindude]

Is your feature request related to a problem? Please describe.
Feature.

MQTT is incredibly useful as an integration tool. Thus:
When an app is loaded into a server, send a MQTT message to a configurable MQTT broker and topic.

[mountaindude]

Looking at something like this in the main config file to deal with outgoing MQTT messages that would be sent whenever an app is cache warmed (=a cache run event takes place)

# MQTT config parameters
mqttConfig:
  out: 
    enable: true              # Should info about cache run/warming events be sent as MQTT messages?
    baseTopic: butler-cw/     # Topic to send cache run events to. Should end with /
    tzFormat: UTC           # LOCAL or UTC. Default is UTC
  # Items below are mandatory if mqttConfig.enable=true
  broker:
    uri: mqtt://<broker-ip>:<port>

[mountaindude]

Will be included in v4.0.0.