Add support for setting Google Pay totalPriceStatus parameter for Unified Checkout

[rwawr]

I'm integrating Unified Checkout with an ecommerce platform that has a single-page checkout form. The user chooses products and enters address/payment information all in one page. I want to show the PANENTRY, GOOGLEPAY, and APPLEPAY checkout buttons when the page loads, which means the capture context must be created ahead of time, but I don't yet know what the final payment amount will be. For PANENTRY and CLICKTOPAY, it works fine to set the amount to $0, get a transient token, and use the token to do an authorization for the final amount once the form is submitted. But for Google Pay (and maybe Apple Pay - haven't tried yet), the user will see $0 as the payment amount, which isn't what they will actually be charged. I know Google Pay has support for setting the totalPriceStatus field to ESTIMATED or NOT_CURRENTLY_KNOWN to provide better UX in these scenarios, and it would be nice if Unified Checkout exposed a way to configure Google Pay (and maybe Apple Pay, which has similar functionality) in this manner. Currently, I'm considering just not allowing Google Pay in this context on my site because the amount of work I'd have to do to get it to show an accurate amount is just not worth it.