feat: simple-pricing plugin

[nnnnat]

Resolves #4970
Impact: minor
Type: feature|refactor

Issue

Pricing data falls under it's own domain within the Reaction Commerce system, however it currently needs to be intertwined and available from other system domains (i.e., Catalog, Cart, Checkout, Orders).

Solution

To give us more flexibility in pricing data management we've begun to move pricing get/set functions into this simple-pricing plugin and calling these functions from the context.queries object from within their respective functions. Now we can fully replace the pricing management system without modification to core by creating a custom plugin that replaces the simple-pricing queries.

Testing

While this acceptance test is testing multiple things it can be done in one pass.
Adding Cart Items

1. Spin up a fresh RC & Storefront with some sample products and payment/shipping enabled.
2. From the Storefront, select a product from the product grid, view the product detail page, select an option and click "add to cart".
3. The MiniCart should pop open with your newly added product displaying the product's price.
4. Repeat the above steps with a different product to see multiple products in the MiniCart.

Price Change
5. From the RC admin select one of the products you've added to your cart in the Storefront.
6. From the products edit screen select the option, change the price then publish the product changes to the catalog.
7. From the Storefront click the cart icon to go to the cart page.
8. From here you should see you're cart item's price has been updated to the new price.

Checkout
9. Now click "checkout" and follow finishes the checkout flow.
10. You should see the correct prices for each item in the checkout review.
11. Place the order, the order should be accepted and you'll be met with the completed order view displaying the products with the prices paid.