Allow crossfade when switching between RGB and colour_temperature modes

Just updated to hass 2021.8 in order utilise the new lighting model I have some smart bulbs setup as RGBWW lights.
When using ```color_interlock: true``` the hass UI displays as I would expect. No extra sliders for colour and whiteness with the separate tabs for colour and white temperature, selecting a colour will switch to it as expected. The only issue I'm having is that I have a 0.5s transition setup on the light as standard for all their changes. Due to the interlock when switching from WW to RGB there's quite a delay as the light has to turn off then turn on the RGB LEDs which results in a moment of total darkness.
I appreciate that the interlock's main purpose is for lights that can't output to both RGB and WW channels at once or due to power limitations but without using interlock it's back to the additional sliders when changing to colour in order for it to display. (which isn't very family/user friendly)
An ideal solution for me would be to have an additional option for interlock such as ```crossfade: true``` which would start the transition to the RGB LEDs at the same time as turning off the WW ones removing the point at which the bulb is completely off. Thoughts?

The request identifies color_interlock, RGB, and color_temperature modes but names no implementation file, test, or entry point. Start by locating where these modes and transitions are implemented, then determine how a crossfade should behave when switching between RGB and white temperature; done means the transition avoids a moment of total darkness while preserving interlock behavior.

Written by the indexing model from the issue text.