R1:64a95000788d

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…