Add adjoints for `(<: Colorants)`

[SomTambe]

Add the following features -

• Adjoint for (CT<:Colorant)(x::T) where T
• Adjoint for (CT<:Colorant)(x::T) where T<:Colorant (Comment)
• Check if #3 (comment) solves the issue in Gradients of constructor Base.ReinterpretArray Zygote.jl#993.