Add support for querying batch product pricing

[fieryprophet]

No issue assigned.

Description

Adds support for querying batch product pricing as defined at: https://developer.bigcommerce.com/api-reference/92c669fa29742-get-prices-batch

Requires customer_group_id and items which is a list of objects specifying the items to price.
Optional parameters are channel_id which defaults to 1 for the default BC storefront ID, and currency_code which defaults to USD.

[r-thomson]

I know BigCommerce structures this resource like /pricing/products, but I think it would make more sense on our end to structure the wrapper like bigcommerce.products.get_prices()