REST API: Migrate order stats

[itsmeichigo]

Closes #8544

Description

Testing instructions

This PR migrates the order stats endpoint to enable REST API. Changes include:

• Updated OrderStatsV4Mapper to parse contents without the data envelope
• Enabled REST API for the loadOrderStats request.

Screenshots

• In DefaultFeatureFlagService, enable the flag for 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 your test store address and tap "Continue".
• Enter the correct site credentials for your account in the test store.
• After the login succeeds, you should be navigated to the home screen.
• On the dashboard screen, notice that the order stats are loaded and displayed correctly.

Please feel free to test again by logging in with a WPCom account. The order stats for your store should still be correct.

---

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

[peril-woocommerce]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 2 days Please, make sure to get it merged by then or assign it to a later expiring milestone

Generated by 🚫 dangerJS

[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 pr8547-7871073 on your iPhone

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

[jaclync]

🚀 - all the reports/revenue/stats requests were successful!