Custom function for button hover+active states

Created by: brneor

I have a specific user case where I'm using custom functions to generate hover and active colors for buttons. This can be only achieved by editing the buttons mixin.

Similar to #32290 (closed), would be nice to have a way to specify the desired function and get options besides various amounts of tint and shade.

Something like

$btn-hover-bg-function:        <current_function> !default;
$btn-hover-border-function:    <current_function> !default;
$btn-active-bg-function:       <current_function> !default;
$btn-active-border-function:   <current_function> !default;