Add compatibility for Multi-Currency and FedEx plugin.

[jessepearson]

Fixes #3128

Changes proposed in this Pull Request

This PR is set to merge into the branch in #3133 due to it uses the same structure described in that PR, along with using some of the code added.

• New WooCommerceFedEx class in the WCPay\MultiCurrency\Compatibility namespace.
  • Uses the wcpay_multi_currency_should_return_store_currency to determine whether to return the store currency or the selected currency.
• Add or update tests as needed.

Testing instructions

• You will need to install the FedEx plugin and set it up.
  • It can be found in the all-plugins repo.
  • The creds for setup can be found in the secret store, and they go under WooCommerce > Settings > Shipping > FedEx.
  • If you haven't set up shipping before, you can add a new shipping zone, and if you don't specify a Zone region, it works for everywhere.
  • Once FedEx is added to your shipping zone, you can set the Origin Postcode to 34480. (It's what I use) https://d.pr/i/RnpOcr
  • Make sure all Services are enabled. https://d.pr/i/XZuTKO
  • That should be it.
• You will need to have a product that has dimensions and a weight set on it. I typically do 5x5x5 for dimensions and a weight of 5.
• Add the item to your cart, and then go to the cart page.
• Check your shipping costs in your store currency against other currencies to make sure to prices are converted correctly.

---

• Added changelog entry (or does not apply)
• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the release testing doc (or does not apply)

[ismaeldcom]

LGTM!