Shipping Labels: Add new entities for WooShippingShipment and Item

[itsmeichigo]

Part of WOOMOB-754

Description

This PR follows up the work in #15885 to add support for persisting Woo Shipping shipments in the local storage.

Specifically:

• Adds new Core Data entities for shipments and shipment items fetched from Woo Shipping config
• Adds a new relationship with shipment to the existing shipping label entity.
• Adds a new relationship between order and shipments.
• Adds conversion to readonly objects.

Testing steps

• Build and run the app and confirm that the app works normally like before without any crashes upon launch.
• Check the setup and relationships of the new entities to confirm that they all look correct.

Testing information

• Quick regression check on simulator iPhone 16 iOS 18.4
• Added tests for Core Data migration.

Screenshots

N/A

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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.

[dangermattic]

	1 Warning
⚠️	This PR is larger than 300 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.

Generated by 🚫 Danger

[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	pr15897-91f1d8b
	Version	22.7
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	91f1d8b
	Installation URL	6n8c5ksmh4r90

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

[RafaelKayumov]

LGTM. Left just 1 question

[RafaelKayumov]

Is it necessary to also store siteID and orderID here since those are accessible through order relationship?

[itsmeichigo]

The order relationship is optional, so I'd prefer having non-optional IDs here instead.