How can I add custom client-side validation to a Webform field using advanced custom properties?
Although this answer does not provide the solution to add custom client-side validation to a Webform field using advanced custom properties, it provides a working alternative to add client-side validation to a Webform field. 

Given that Webform UI comes with the option to add custom CSS and Javascript to a Webform, I have made use of this option to add my custom validation.