Is your feature request related to a problem? Please describe.
This is a follow up to #566 at the request of @loranallensmith

There is explicit Product Addons support built in now it seems, but otherwise, there's currently no way for the request buttons to work on the single product page for more advanced product types or customized products. This impacts 2 of my plugins: Name Your Price and Mix and Match Products...(as well as Product Bundles, Composites, and probably still some of the other plugins listed in #566)

For example, in Name Your Price... a user enters a price and that becomes the price of the product, which is currently not possible. So there are several blockages for working with the payment request buttons:

1. Customer can click the buttons when the price entered is not valid (therefore checking out without meeting the minimum... or in the case of Mix and Match when the container's contents are not selected)
2. There's no way to pass the form inputs via the ajax request to the backend. Product Addons fields are hard-coded.
3. There's no way to modify the price as a new product object is instantiated and is added to cart without any cart params

Describe the solution you'd like
I think multiple things need to happen to make this a flexible framework that can be extended as needed by other plugins. This is just the minimum things I can think right now:

1. Enable/disable the request buttons when the product form is not validated. (Very cursory start here)
2. Customize the popup alert message that is triggered when the product cannot yet be added to the cart
3. serialize all <form> inputs to relay via ajax
4. Some kind of filter in ajax_add_to_cart() that allows for the manipulation that allows for the manipulation of $cart_item_data param in add_to_cart() *This may not be necessary... would need to see if the hooks in add_to_cart() itself are sufficient if the form data gets passed along in the $_POST but seems like to could be helpful.

Describe alternatives you've considered
I've added code to my plugins to explicitly disable the payment request buttons for products using my plugins, which seems a shame because that's a cool feature. I am willing to write the compatibility code on my side but currently there's no framework to make that possible.

While cross-compatibility is never fun, it's my opinion that compatibility between plugins in the marketplace is a huge selling point.