Order Fulfillment: Custom tracking providers to be stored locally

[anitaa1990]

(Copied from the iOS issue: woocommerce/woocommerce-ios#945)
At the moment, when a user adds a custom shipment tracking, the name and url of the custom shipment tracking provider are not stored on the backend or the app.

This matches the behaviour on web.

However, for a user in a country that is not included in the list of shipment tracking provided by the plugin API (i.e. Hong Kong) adding shipment tracking will require entering the name and url of their favourite provider again and again, order after order.

We could explore storing that information locally and pre-populating the form used to enter custom trackings with those values:

Screenshot of the issue:

[joshbetz]

@anitaa1990 Can you help me understand what changes are needed in the API to support this?