Allow setting underglow hue, saturation and brightness separately (or specifying the number of steps for HSB increments/decrements)

[why-trv]

Why? For example, I'd like to use underglow color to indicate the current layer. Currently, I can only set HSB altogether - and it kinda works, but I'm no longer able to adjust brightness to ambient light conditions, I have to stick with a single brightness level that can be either too little or too much.
I guess it may be doable using RGB_HUD and RGB_HUI with relatively large CONFIG_ZMK_RGB_UNDERGLOW_HUE_STEP, but that feels like a workaround. Adding a number of steps parameter to those actions could work though.