Add net and gross purchase prices

[hanneswernery]

This is an updated and improved version of #663 for the 6.3.1.0 release

1. Why is this change necessary?

Currently the product and the payload of the order line items have a purchase price field of type float. Many users want to manage their purchase prices as net prices. Some want to use different currencies when purchasing products.
Hence the more complex PriceField with gross and net prices as well as currencies would be more suitable.

2. What does this change do, exactly?

Adds a new purchase price field of the more complex type PriceField which supports purchases prices as net and gross prices as well as purchase prices in multiple currencies (even though the code right now only uses a single purchase price in the default currency).

• A field purchase_prices of type PriceField is added to the product definition as well as to the order line items (and some corresponding classes).
• The old float purchase price field (and some corresponding functions) is marked as deprecated.
• The existing LineItemPurchasePriceRule improved to handle gross or net purchase prices. Existing rules are migrated.
• A migration is added to migrate the deprecated purchase price field to the new one.
• An ApiConverter is added to provide the old purchase price field for the api v2 and the new field for the api v3.
• A database trigger is added to keep the deprecated purchase price field in sync with the new one (updating either will update the other).
• The administration product view is updated to display the new purchase price field.
• Existing PHPUnit and Cypress e2e tests are updated and some new tests are added.

3. Describe each step to reproduce the issue or behaviour.

4. Please link to the relevant issues (if any).

• https://issues.shopware.com/issues/NEXT-7666
• https://issues.shopware.com/issues/NEXT-5021

5. Checklist

• I have written tests and verified that they fail without my change
• I have squashed any insignificant commits
• I have written or adjusted the documentation according to my changes
• This change has comments for package types, values, functions, and non-obvious lines of code
• I have read the contribution requirements and fulfil them.

[mitelg]

thanks for your really awesome contribution @hanneswernery 👍 🎉 💙