[Shipping Lines] Add Networking support to fetch shipping methods

[rachelmcr]

Closes: #12579

Why

We want to fetch a list of the available shipping methods on a store, so the merchant can select a method when adding shipping to an order. This adds networking support to fetch the shipping methods.

How

• Adds a ShippingMethod networking entity with the method ID and title.
• Adds ShippingMethodsRemote and a loadShippingMethods(for:) method to fetch the methods from remote.
• Adds a ShippingMethodListMapper to map the response from the remote endpoint.

Testing instructions

This endpoint is not yet used in the app; confirm unit tests pass.

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

[dangermattic]

	1 Warning
:warning:	This PR is assigned to the milestone 18.5. 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

[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	pr12632-4ed90db
	Version	18.4
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	4ed90dbf0cfc7fe01283086b9507ec6649ebe0f2
	App Center Build	WooCommerce - Prototype Builds #8927

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