[Feature request] - Add native support of virtual switch in over_switch VTherm

[jmcollin78]

For devices like Nodon SIN-4-FP-21 or Heatzy or other specific devices, the integration of VTherm needs a virtual switch lmiek explained here: https://github.com/jmcollin78/versatile_thermostat/blob/main/documentation/en/troubleshooting.md#troubleshooting

This virtual switch is a mess for beginners because it should be done in yaml code exclusively, need restart each time it is update.

This feature request aims to provide a way to support the virtual switch configuration directly in VTherm.

The idea is to provide:

1. a way to specify the start-up command of the underlying device,
2. a way to specify the stop command of the underlying device

Manny examples can be found here: https://forum.hacf.fr/t/versatile-thermostat-integration-native-des-nodons-et-heatzy/56438/22