Create a simple implementation for websockets

[fhessel]

The server should support websockets to allow browser-based APIs to adjust values in a streaming manner without interruptions due to resource resolving etc.
This could be used to control outputs of the ESP32 in real time, e.g. a PWM-enabled pin or to stream readings from the ADC to the application without big overhead.

A possible way of implementation could be to create a new type of callback function, that is not called just once to handle a specific request, but several times during each call of the HTTPSServer's loop() function.

[fhessel]

Duplicate of #13