[Shipping labels] Add shipping label purchase action to WooShippingStore

Closes: #14424 Closes: #14053 ⚠️ Depends on https://github.com/woocommerce/woocommerce-ios/pull/14488 ⚠️

Description

This adds an action in the Yosemite layer to purchase a shipping label. It uses the endpoints for purchasing a shipping label and polling for a shipping label status, to check when the purchase is complete and return the purchased label.

How

Here is the polling strategy we're using:

This is essentially the same action we already have in ShippingLabelStore, with some small adjustments due to differences in the new endpoint request/response schemas.

Testing information

TBD: I plan to test this action before merging it, to confirm the actual requests/responses work as expected.

[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.
[ ] 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	pr14490-e0aaa21
	Version	21.1
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	e0aaa2122714e64fdec13b1ba7ccd98781ff87f2
	App Center Build	WooCommerce - Prototype Builds #11719

woocommerce / woocommerce-ios