The rsync command currently used in the celo-migrate script can be run as part of the pre-migration step, without applying any transformations or state changes on top of it. It seems the --update and --delete flags can be used to only rsync new data during the actual migration.

### Tasks
- [x] Add pre-migrate sub-command
- [x] Have rsync run as part of pre-migration with proper flags
- [x] Consider adding back state-dry-run functionality