Extend Homeassistant discovery with button for flow start

[caco3]

• ctrl/flow_start
• Remove if (_data_len > 0) { , see Publish GPIO settings with MQTT is not included in HA auto discovery #3302 (comment)

ctrl/flow_start

Adds a button to to Homeassistant to trigger the flow:

out of scope for this Pull request:

• ctrl/set_prevalue
• GPIO/GPIO{PinNumber} See See Publish GPIO settings with MQTT is not included in HA auto discovery #3302
  -> but only those pins which have MQTT enabled

[caco3]

@Slider0007 I tested your code for the flow control start and it seems to work well.
I looked into how to do it with the set_prevalue, how ever the problem seems to be that we can't pre-configure HA to show a useful input. I tried it with a homeassistant/number/ (see #3416), how ever the shown slider is not useful at all:

See also https://community.home-assistant.io/t/mqtt-number-optional-mode-for-slider-box/362880

One would have to implement it with 2 elements like shown here: https://community.home-assistant.io/t/prompting-for-input-in-a-script/602702/4. But i doubt this is possible with the discovery protocol.

Therefore I would keep the set_prevalue out of scope.

The GPIOs should be feasible, but I haven't looked into it yet. Ideally on the HA UI they would represent the real state.