Decouple some model implementations from the app targets

[mokagio]

Extracts some types and extensions in dedicated files to decouple from usages of types that belong to the apps targets and that we are not yet ready to move to WordPressData as part of #24165

Similar to #24326 but with the difference that these changes are not simple public additions, therefore I felt it better to make them in dedicated commit and in a standalone PR for ease of review.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 25.9. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr24331-3e0f7d5
	Version	25.8
	Bundle ID	org.wordpress.alpha
	Commit	3e0f7d5
	App Center Build	WPiOS - One-Offs #11866

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

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr24331-3e0f7d5
	Version	25.8
	Bundle ID	com.jetpack.alpha
	Commit	3e0f7d5
	App Center Build	jetpack-installable-builds #10889

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