Enable/Disable Standby of HeidelbergWallbox

[mich273-art]

Created a new MQTT topic /standby_enabled to control the Standby register of the Heidelberg Wallbox. The topic is automatically discovered by Home Assistant.

Tested successfully on the LILYGO build:

• MQTT topic HeidelBridge/standby_enabled is published correctly.
• Home Assistant controls added.
• Standby mode can be set via Home Assistant → Wallbox enters deep standby after ~10-15 minutes.
• Wakeup via Heidelberg interface is not possible while in deep standby.
• Wallbox wakes up automatically when a car is connected, and control via Home Assistant/MQTT is restored.
• Disabling Standby also works.

EVCC will not work if Wallbox is in deep standby.

[Giermann]

I don't have any Heidelberg boxes to test, so this is only a dry review:

Unlike mChargingEnabled/IsChargingEnabled() the introduced mStandbyEnabled is written to registers inside the Wallbox and persists reboots of the HeidelBridge!
So I'd suggest to rename the method from IsStandbyEnabled() to GetStandbyEnabled() and to use ReadRegisters to get the current status instead of statically set it to true on initialization.

Has anyone measured how quick the Wallbox wakes up, starts charging and reconnects to Home Assistant?
Maybe it would be help to set a lower Charging Current before the box enters deep standby, maybe on every disconnect of the car. This avoids charging with too high currents after wakeup, if the reconnect to HA is too slow. But this should be handled in another issue or PR.

[mich273-art]

Unlike mChargingEnabled/IsChargingEnabled() the introduced mStandbyEnabled is written to registers inside the Wallbox and persists reboots of the HeidelBridge! So I'd suggest to rename the method from IsStandbyEnabled() to GetStandbyEnabled() and to use ReadRegisters to get the current status instead of statically set it to true on initialization.

Thank you for your review and suggentions!
I tried to implement it accordingly.

[mich273-art]

Observation from today:
After Wallbox ist in Deep Standby and a Car gets connected the Charging Current is 0.
That means charging does not start immediatly.
Only after selecting a currunt (Home assistant controls) IT will start charging.