[Blaze] Enhance loading UI for product image in Blaze Campaign Creation form

[pmusolino]

Fixes #15094
While creating a new Blaze Ad Campaign, during the loading process, you might notice a large t-shirt on the transition screen. This t-shirt serves as the placeholder for the campaign image in case there is no image associated with the product.

So in this PR I

• Updated BlazeCampaignCreationForm to add redacted and shimmering effects when loading product image.
• Updated isLoadingProductImage visibility in BlazeCampaignCreationFormViewModel to be read-only and set default to true since the image will be fetched async.

Steps to reproduce

1. Ideally start with a clean installation, so you don't have images in cache.
2. Use a store that is eligible for Blaze
3. While creating a new Blaze ad campaign, notice that the big t-shirt icon in the transition.

Testing information

Repeat the steps to reproduce, and you should now see a redacted and shimmering effect on the loading product image, or a placeholder t-shirt image if there is no image to load.

Video

The error alert in this video appears because the AI suggestion service was offline while recording the video. C069RG07W6S/p1738935520270549

fix.issue.blaze.loading.mp4

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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.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• 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	pr15098-c6a5deb
	Version	21.7
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	c6a5deb
	App Center Build	WooCommerce - Prototype Builds #12883

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

[itsmeichigo]

Tested on simulator iPhone 16 Pro iOS 18.2 and confirmed that the glitch is no longer present.