CLI download improvements

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/12147

This PR:
- extends the CLI download plugin to support all the following forms:

  ```
  bin/omero download file_id target
  bin/omero download OriginalFile:file_id target
  bin/omero download FileAnnotation:fa_id target
  bin/omero download Image:image_id target
```
- adds integration tests for the `omero download` command checking the validity of the forms above.

When an image is passed with `Image:id`, the original file linked to the fileset is retrieved and downloaded. Integration tests currently test this with a fake MIF file and the `dv` image used in the testing suite. Untested cases so far are:
- what happens with multiple original files/companion files? Do we have ways to simulate this in the test suite /cc @ximenesuk ? More generally, how would we modify the signature of the `download` command?
- what happens for pre-FS data? Same as above, is there an existing way to simulate it and integrate it in the test?

To test this PR:
- check the new integration tests pass in the daily merge integration job
- check the various forms of the `omero download` described above. Focus especially on different types of images (pre 5.0, formats, MIF...)