[verb] `copy` - to copy from one table to another with a hook to transform

If you want to move data from one table to another and possibly manipulate the items as they go, that's something we could provide with a copy verb.

```
./bulk copy --from t1 --to t2 --transformer trans.py
```

The transformer could limit what gets sent, reshape it, redact data, etc.

If there's no need for the transform, a backup/restore would be sufficient, so this is really about a transformational copy. A better name might be `transform` but that feels like it would be in-place.

An export, manipulate the content in S3, and import to a new table is another option. This would be more straightforward.

Start with the proposed `./bulk copy --from t1 --to t2 --transformer trans.py` entry point and compare it with the stated export, S3 manipulation, and import alternative. Done means the project has a decided transformational-copy design, including how the transformer participates; the issue names no implementation files or tests.

Written by the indexing model from the issue text.