[Woo POS][Product Labels] Display stock levels for variations

[iamgabrielma]

Closes WOOMOB-495

Description

As continuation of #15647 , this PR adapts the POSVariation model to use manageStock, stockQuantity, and stockStatusKey properties. As well as render them on the view (final UI TBD)

Testing

• Create a variable product with different variations in different stages of managed and non-managed stock. For simplicity when testing you can load POS for https://indiemelon.mystagingwebsite.com/ and look for "Variable product with managed stock" product, which contains these.
• Observe that a variation shows:
  • If not managed, just In Stock or Out of Stock.
  • If managed, the number of items in stock, or Out of Stock.

• You can play with those in wp-admin and exit/enter POS or PTR to fetch the latest state. This is under Products > "Variable product with managed stock" > Edit > Variation > Click on the row to edit stock status or stock levels

• When feature flag inventoryProductLabelsInPOS is disabled we shouldn't see any changes in the variation cards

---

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[iamgabrielma]

StockStatusKey happens to be in Networking at the moment, so we cannot use it directly in this target. I'll update the usage from String to an enum with WOOMOB-517 when testing/handling undocumented stock statuses.

[iamgabrielma]

Actually we can, not sure why Xcode was not finding it on a first go, as I can see the typealias it in Yosemite and compiles if I switch it despite the warnings.

[wpmobilebot]

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

	App Name	WooCommerce iOS Prototype
	Build Number	30091
	Version	PR #15676
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	3130e26
	Installation URL	4is8l2q7qr6e0

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

[iamgabrielma]

No need to review this one for the moment @staskus as we might need to move this project for the time being. I'll switch it back to draft for now, and close it in a few days if we go ahead with the decision 👍

[wpmobilebot]

Version 22.5 has now entered code-freeze, so the milestone of this PR has been updated to 22.6.