Add feature flag for the product global unique identifier

[toupper]

Closes: #14160

Description

This PR adds a feature flag to hide the implementation of the Product Global Unique Identifier while its implementation

Steps to reproduce

N/A

Testing information

N/A

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.

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• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.

[wpmobilebot]

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

	App Name	WooCommerce iOS
	Build Number	pr14169-97b478e
	Version	20.8
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	97b478e
	App Center Build	WooCommerce - Prototype Builds #11198

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