Support seeding a new repository from an existing one

[vkamra]

This feature would allow a user to initialize a new Corso repository and seed it with an existing backup from a different repository.

Seeding it with an existing backup would allow new backups to continue as incrementals.

[vkamra]

If we added a generic backup migrate command to support this use case - it would also enable other use cases that require backup copies (moving storage providers, etc.)