Provide multiple ways to generate VAT prices in core

[kennyadsl]

Is your feature request related to a problem? Please describe.

#3448 outlined that it's quite common to have the need for a constant gross price across several countries when calculating VAT prices.

The current behavior of the vat prices calculator with tax rates that have the included_in_price value is:

• fix a net price amount (the actual default price of the variant)
• for each country requiring VAT (ones that have included_in_price tax rates in that variant's tax_category) create a price that is net price amount * (1 + tax rate amount)
• during the checkout, the VAT amount will not be added to the order total, it's already part of the price, but it will be shown as "already present" for taxing laws requirement.

There are stores that need to generate prices differently though. The fixed part should be the gross price amount, which should be identical in all countries.

Describe the solution you'd like

We already provided a way to change with #3451 but as @mamhoff pointed out, we could add this class in core allowing people to select it without adding any code.

Describe alternatives you've considered

I was wondering if we can make this thing more granularly, allowing admin users to select which method to apply per tax rate or per country?