Please support user defined data channels

[zapta]

Example: user wants to hook a VOC sensor (e.g. via i2c) and display its value over time graph in the DWC.

One possible approach

1. User can define a numeric data point. e.g. name, min, max.
2. User provides data points. Can be push mode where the user provide with gcode <point, x, y> or pull where RRF samples a give gcode variable
3. DWS pulls the time serie and displays it as current value and a graph.

Inputing the data to the RRF can be done via existing inputs (I2c, serial, digital inputs, etc) and processed/scale by the user (e.g. in a daemon job) to have the channel's value.