[Shipping Lines] Add Yosemite support for fetching and storing shipping methods

[rachelmcr]

Closes: #12612

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 Yosemite support to fetch and store the available shipping methods.

How

• Adds a ReadOnlyConvertible extension for ShippingMethod, to convert shipping methods to/from the Storage layer.
• Adds ShippingMethodAction and ShippingMethodStore with an action to sync and store the shipping methods from remote. This will allow us to populate the shipping method list in the UI layer from storage.
• Adds ShippingMethodStore to AuthenticatedState.

Testing instructions

This new action/store 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.

[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	pr12642-fd1babe
	Version	18.4
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	fd1babed4fce80db912a08092c65452360d20d02
	App Center Build	WooCommerce - Prototype Builds #8937

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