REST API: Migrate the data remote for the country list

[itsmeichigo]

Closes: #8583

Description

This PR continues the migration work for REST API:

• Updated the CountryListMapper to parse content without the data envelope
• Enable REST API on the country list fetch endpoint.

Testing instructions

• Enable the feature flag applicationPasswordAuthenticationForSiteCredentialLogin and build the app.
• Log out of the app or skip onboarding if needed.
• On the prologue screen, select "Enter your site address" and enter the address of your self-hosted store.
• Proceed to login with site credentials.
• After the login succeeds, you should be navigated to the home screen.
• Select the Orders tab and tap "+" to add a new order.
• On the order creation form, select "Add Customer Details" > "Country" to add a country for the user.
• Notice that the country list is loaded properly.

Screenshots

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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 by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr8584-dd154de on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[selanthiraiyan]

LGTM 🎉