Feature request: Fan mixer

[ogland]

M106 commands acting like a mixer where it mixes in something like aux fan at a certain point in the progression of the S param.

Slicers like Orca aux fan command is so very basic, for instance setting a fixed fan percentage starting from a fixed layer height. I think we could gain more value from it if it could act more like the cream on top of any toolhead fan.

[levins-law]

This could be a related function to the heated part cooler problem I introduced in Discord