Shipping Labels: Optimize country syncing requests

[itsmeichigo]

Closes WOOMOB-771

Description

Currently we're syncing countries for every order item in the customs form and in the address form as well. The country list is unlikely to change so often, so making these requests so frequently is unnecessary.

Since both customs form and edit address form are getting the country list from storage, syncing the list with the remote once when starting the purchase flow should be sufficient.

This PR optimizes the requests by moving the syncing request to the purchase form and remove the requests from the other forms.

Testing steps

1. Remove the Woo app from your simulator/device if it's already installed to clear all cached data.
2. Install the app and log in to a test store with Woo Shipping set up.
3. Navigate to the Orders tab and select an order eligible for creating shipping labels with multiple items.
4. Update the shipping address to an address outside of the US.
5. Open the purchase flow and confirm with Proxyman that the country list is fetched only once.
6. Open a customs form and confirm that the country list is correct.
7. Open the edit address form for the origin or destination address.
8. Confirm with Proxyman that the country list is not fetched again. Confirm that the country list is correct.

Testing information

Tested with simulator iPhone 16 iOS 18.4.

Screenshots

No UI changes.

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS Prototype by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS Prototype
	Build Number	pr15875-3fdaad5
	Version	22.7
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	3fdaad5
	Installation URL	39upoof2hrs5g

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[RafaelKayumov]

LGTM. Works as described. The countries list looks complete in the app. The countries list /wc/v3/data/countries&_method=get fetched only once according to Proxyman.