Make check label cursor customizable (#29633)

scss/_custom-forms.scss

@@ -76,6 +76,7 @@
   margin-bottom: 0;
   color: $custom-control-label-color;
   vertical-align: top;
+  cursor: $custom-control-cursor;
 
   // Background-color and (when enabled) gradient
   &::before {

scss/_variables.scss

@@ -526,6 +526,7 @@ $custom-forms-transition:               background-color .15s ease-in-out, borde
 
 $custom-control-gutter:                 .5rem !default;
 $custom-control-spacer-x:               1rem !default;
+$custom-control-cursor:                 null !default;
 
 $custom-control-indicator-size:         1rem !default;
 $custom-control-indicator-bg:           $input-bg !default;