[Shipping labels] Add support for Woo Shipping label purchase endpoint in Networking layer

[rachelmcr]

Closes: #14423 ⚠️ Depends on #14455 ⚠️

Description

This adds support in the Networking layer for purchasing a label in the Woo Shipping label flow.

How

• Adds a WooShippingPackagePurchase to represent the package data needed in the purchase request.
  • Includes custom encoding for the packages request parameter.
  • Includes custom methods to encode additional request parameters (rate, hazmat, customs) with the shipment ID.
• Updates ShippingLabelCarrierRate and ShippingLabelCarriersAndRates to handle different key strategies for decoding and encoding:
  • Previously, these were only decoded from responses, with snake case keys. (Both models were marked as Codable but we didn't encode them directly for any requests.) However, the purchase request requires encoding these entities with camel case keys.
  • The coding keys are now camel case by default (to support encoding), with a key decoding strategy (set in the corresponding mappers) to convert to snake case.
• Adds the purchase endpoint to WooShippingRemote, along with the corresponding mock response and unit tests.

A note on the number of changes in this PR: 200+ lines of changes are from testing mocks and generated fake/copy methods for unit testing. I considered trying to split up some of the changes to the networking models here into a separate PR from the WooShippingRemote changes, but I think these changes make more sense in context.

Testing information

Testing steps to confirm these changes don't change the legacy shipping labels flow:

1. Disable the feature flag revampedShippingLabelCreation.
2. Build and run the app.
3. Go to the Orders tab and select an order eligible for shipping label creation.
4. Tap "Create Shipping Label."
5. Go through each step of the shipping label flow and purchase a shipping label, confirming there are no errors and the label can be viewed/printed 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.
• [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.

[dangermattic]

	2 Warnings
:warning:	This PR is larger than 300 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.
:warning:	This PR is assigned to the milestone 21.2. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

Generated by :no_entry_sign: Danger

[rachelmcr]

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

Just a note that 200+ lines of changes are from testing mocks and generated fake/copy methods for unit testing. I considered trying to split up some of the changes to the networking models here into a separate PR from the WooShippingRemote changes, but I think these changes make more sense in context.

[wpmobilebot]

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

	App Name	WooCommerce iOS
	Build Number	pr14474-997e863
	Version	21.1
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	997e86397d70a8c56b53d1e6817bc51db5a908d6
	App Center Build	WooCommerce - Prototype Builds #11716

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