add mqtt output plugins for SBFspot and ahoyDTU

[chfritsch]

This commit adds output plugins to provide the same mqtt data like

• SPFspot (https://github.com/SBFspot/SBFspot)
• ahoyDTU (https://ahoydtu.de)

[kbialek]

@chfritsch Thank you for this PR. I've added the plugin support to let the users to extend the functionality with code that is not kept in this repo and therefore not maintained by me. If you wish to share your work with the community, please create your own github project, then let me know the project url, so I can add it in the README.

[tobit6795]

i wonder if it makes sense to extend this with this write topic {MQTT_TOPIC_PREFIX}/settings/active_power_regulation/command. As far as i can see, the only "problem" is, that we can only send values with max one decimal place, means for an 800w inverter i cannot set the output to 235W which would be 29,375% / 2.9 ~ 232W

[hoegaarden]

@chfritsch If you want to, I am happy to accept PRs to https://github.com/hoegaarden/deye-inverter-mqtt-plugins/, which I set up to be some sort of dumping ground for various plugins.