Add 'get_supported_currencies' function for klarna

[Mayisha]

Fixes #3351

Changes proposed in this Pull Request:

Added get_supported_currencies for Klarna which returns the list of supported currencies based on the Stripe account currency.

Testing instructions

• Connect your store to an account from the US.
• Change your store currency to EUR.
• Enable Klarna under the Payment Methods tab in the plugin settings.
• As a shopper, add a product to your cart and go to the checkout page.
• Ensure your billing country is the United States.
• In develop branch notice that Klarna is displayed. When you select it, the Payment Element isn't displayed. The following error shows up in the browser console.

• In this branch, Klarna is not displayed as the store currency is EUR.
• Confirm that there is not regression around the fix related to EEA cross border transaction.

[Mayisha]

@james-allan I have requested your review here as you have the most knowledge in Klarna. As you had fixed the previous issue #3108, you are the best person to save me from creating any regression. 🥺

[james-allan]

Nice work on this @Mayisha. The super detailed function comment is really nice at explaining the various scenarios. It should help us understand how it currently works and where problems might be if our understanding is incorrect.

Based on scenarios outlined in the comment and my understanding of Klarna requirements I tested the following matrix of set ups and customer situations:

Acc	Store Currency	Customer location	Klarna offered (👍)
🇺🇸	US	US	Yes (👍)
🇺🇸	US	AU	No (👍)
🇺🇸	AU	AU	No (👍)
🇺🇸	AU	US	No (👍)
🇦🇺🐨🦘	AU	AU	Yes (👍)
🇦🇺🐨🦘	AU	US	No (👍)
🇦🇺🐨🦘	EUR	FR	No (👍)
🇦🇺🐨🦘	EUR	AU	No (👍)
🇫🇷	EUR	FR	Yes (👍)
🇫🇷	EUR	Germany	Yes (👍)
🇫🇷	EUR	Greece	Yes (👍)
🇫🇷	EUR	UK	No (👍)
🇫🇷	EUR	Switzerland	No (👍)
🇫🇷	EUR	US	No (👍)
🇫🇷	NOK	Norway	Yes (👍)
🇫🇷	NOK	FR	No (👍)
🇫🇷	AU	AU	No (👍)
🇫🇷	AU	FR	No (👍)
🇬🇧	GBP	GB	Yes (👍)
🇬🇧	GBP	FR	No (👍)
🇬🇧	GBP	US	No (👍)
🇬🇧	AU	AU	No (👍)
🇬🇧	AU	GB	No (👍)
🇬🇧	NOK	NO	Yes (⚠️)

• ^{⚠️ This case I wasn't 100% sure but they work and appear correct based on the final case listed in the advanced rules of EEA countries.}
• ^{👍 Indicates that the expected outcome occurred.}
• ^{Bolded rows are the outcomes where Klarna was shown to customers on the checkout - every other case was hidden.}

[Mayisha]

Thank you @james-allan for testing so many scenarios and documenting them 💯