R1:af5769a6be28

Add a "--copy" flag to "bin/files migrate"

Summary:
Ref T11596. When exporting data from the Phacility cluster, we `bin/files migrate` data from S3 into a database dump on the `aux` tier.

With current semantics, this //moves// the data and destroys it in S3.

Add a `--copy` flag to //copy// the data instead. This leaves the old copy around, which is what we want for exports.

Test Plan:
- Ran `bin/files migrate` to go from `blob` to `disk` with `--copy`. Verified a copy was left in the database.