Allow admins to enter a custom shipping rate on the shipments form

Problem/Motivation

To the best of my knowledge, there is no way to enter a custom shipping rate on the shipments form. In Commerce 1 there was a "Manually specify a shipping service and rate" radio option which allowed an admin to the select the shipping service and enter a custom rate (Please see attached).

Proposed resolution

Add a radio option similar to Commerce 1 or a new widget to the shipments form to provide this functionality.