FR: Loosen up the templating of product_variants tag

[ncla]

Right now if you want to change the markup of the <select> coming from product_variants tag then you must create basically another tag and duplicate this addons tag code. The methods responsible for creating the markup are private, and I cannot simply extend the ProductVariants tag class and just override parseOptions, where I could add the HTML attribute I want. Ideally, in my opinion, this templating should be moved to antlers or blade templates.

[jackabox]

@ncla Sure, it's meant to be quite tight at the moment but I could see the use case. Happy for a PR to tackle this.