REST API: Migrate refund endpoints

[itsmeichigo]

Closes: #8568

Description

This PR migrates the refund endpoints with the following changes:

• Updated the RefundMapper and RefundListMapper to parse contents without the data envelope.
• Enabled REST API on all refund endpoints.

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".
• Enter the address of your self-hosted site and proceed to log in with site credentials.
• After logging in, navigate to the Orders tab. Select an existing order or create a new one if needed.
• Update the order status to "Completed", then select "Issue Refund" on the order detail screen.
• Create a refund for the order. Notice that the creation succeeds.
• Navigate back to the order list and select the order again. Notice that the created refund is listed in the order detail screen.
• Select the refund to see its detail. Notice that the detail can be loaded correctly.

Please feel free to test again by logging in to a WP.com account. The refund feature should still work correctly.

Screenshots

N/A

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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 pr8569-99e9656 on your iPhone

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

[selanthiraiyan]

LGTM 🚢

[peril-woocommerce]

	Warnings
⚠️	PR is not assigned to a milestone.

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