Feature reqest: color image to grayscale one with gradient map

[kirk23]

With a set amount of colors /gradient stops.   So as input we have just RGB image then the filter turns it into grayscale one  + gradient map filter .    Thus we would  have color info stored in one channel  and a gradient.      Working like sort of indexed colors.  So we say  the gradient should have 100 stops  for example  and it makes 100 gradation of gray with corresponding stops in the gradient.