CheckboxInput: Allow custom icon

[Anlerkan]

I think we should be able to customize the checkbox input icon

• We can handle this by adding customIcon prop to CheckboxInput

export interface CheckboxInputProps {
  item: RadioInputItem;
  onSelect: (
    item: RadioInputItem,
    event?: React.SyntheticEvent<HTMLInputElement>
  ) => void;
  isSelected: boolean;
  isDisabled?: boolean;
+ customIcon?: React.ReactNode;
  customClassName?: string;
  testid?: string;
}

const icon = customIcon || <CheckIcon />;

<span className={"checkbox-input-label__icon"}>{icon}</span>

• In addition, we should also take into account the icon's visibility. It should be visible only when isSelected is true.