Add "bin/file migrate" options to support import of a local-disk backup for Phacility instances

Summary:
Ref T13306. Currently, there's no easy way to import a third-party local-disk file dump into a Phacility instance.

Add some more options to bin/files migrate to support this. In particular, this enables:

$ ./bin/files --from-engine local-disk --engine amazon-s3 --local-disk-source path/to/backup

...to import these files into S3 directly.

These are general-purpose options and theoretically useful in other use cases, although realistically those cases are probably very rare.

Test Plan: Used bin/files with the new options to move files in and out of local disk storage in an arbitrary backup directory. Got clean exports/imports.

Reviewers: amckinley

Maniphest Tasks: T13306

Differential Revision: https://secure.phabricator.com/D20571