[Woo POS] Extract onboarding to aggregate model

Closes: #14452

Description

This PR moves onboarding to the new Aggregate Model.

Testing information

I haven't tested every single onboarding state; the logic is simply moved, but otherwise unchanged, so it didn't seem neccesary to test every possible state.

I've gone through a new store flow as shown in the video below. It's very long! Everything worked as expected, however I'm not sure whether keeping it in a non-fullscreen sheet is the best when using WP-Admin. On the other hand... it's never going to be a good experience doing it in a web view.

The most important things to check are:

the display of onboarding,
cancellation (via the x and tapping outside the modal)
actions
analytics

Screenshots

https://github.com/user-attachments/assets/e652aec1-18ef-478f-8afe-acd37e5a0370

[x] 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:

[x] 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.
[x] Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
[x] 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.

	App Name	WooCommerce iOS
	Build Number	pr14456-0b1e3b8
	Version	21.1
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	0b1e3b84024eec945644dc98fd26f8510a69cfa6
	App Center Build	WooCommerce - Prototype Builds #11655

woocommerce / woocommerce-ios