[Shipping Lines] Add shipping methods tracks events

[rachelmcr]

Closes: #12580

Description

We want to track 3 events around shipping method selection:

• Updated event:
  • order_shipping_method_add with the custom property shipping_method containing the id of the selected shipping method in lowercase (ups|flat_rate|free_shipping…).
  • Event registration: https://github.com/Automattic/tracks-events-registration/pull/2400
• New event:
  • order_shipping_method_selected, triggered every time a merchant selects a new shipping method.
  • This event contains a custom property shipping_method with the id of the selected shipping method in lowercase (ups|flat_rate|free_shipping…).
  • Event registration: https://github.com/Automattic/tracks-events-registration/pull/2401
• New event:
  • order_add_shipping_tapped, triggered every time a merchant taps on the add shipping button.
  • Event registration: https://github.com/Automattic/tracks-events-registration/pull/2402

Testing instructions

1. Go to the Orders tab.
2. Create a new order.
3. Add a product to the order.
4. Tap the "Add Shipping" button and confirm the order_add_shipping_tapped event is tracked.
5. On the shipping screen, tap "Method" to open the shipping methods list.
6. Select a new method and confirm the order_shipping_method_selected event is tracked with the custom prop shipping_method with the correct shipping method ID.
7. Add a shipping amount.
8. Tap "Add Shipping" to add the shipping method to the order, and confirm the order_shipping_method_add event is tracked with the custom prop shipping_method with the correct shipping method ID.

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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 18.6. 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.

	1 Message
:book:	This PR contains changes to Tracks-related logic. Please ensure (author and reviewer) the following are completed: The tracks events must be validated in the Tracks system. Verify the internal Tracks spreadsheet has also been updated. Please consider registering any new events. The PR must be assigned the category: tracks label.

Generated by :no_entry_sign: Danger

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	App Name	WooCommerce iOS
	Build Number	pr12686-4211f36
	Version	18.5
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	4211f36f4e91b35716c09bec1f9128a55dca6246
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