Add an API for just product inventory and price

[ScopeyNZ]

The advice for Commerce.js advocates that websites take advantage of static rendering to ensure that product display pages are fast and lightweight to render, as well as getting other benefits from static rendering like no-js support. Unfortunately, the price and stock level of a product isn't so static, so it can be difficult to combine static rendering with this non-static information.

Commerce.js & the Chec API should provide an API (or APIs) to fetch just the invetory and/or price of products so that product information can be pulled statically, and the same information isn't loaded for requests that are only concerned about price & inventory.