Add restore-external-snapshot Target

This adds a make restore-external-snapshot target leveraging the new format provided by !962 (merged) / !963 (merged) (open-source code to create these snapshots, so that entities beyond Nine Realms can more easily become providers). The default is still Nine Realms, but it prompts for the endpoint, so another can be passed. Once the new service has a few snapshots under its belt we'll deprecate the recover-ninerealms target in favor of this.