Make disk usage plugin work minimally against new response.

This revision of the disk usage plugin is primarily designed to test gh-3271  It sums basic usage and adds user and group columns to the report form. With gh-3271 the plugin can be expanded to aggregate data either in this PR or as a separate PR post-merge.

--depends-on #3271
--no-rebase

Testing
=======
A simple test would be to run
```
bin/omero fs usage --report
```
as a user who is in several groups. This should give a tabular breakdown of disk usage as per https://github.com/openmicroscopy/openmicroscopy/pull/3009#issue-41967453 but with additional columns containing user and group IDs.

Without the `--report` option the result should be a single number that is the sum of whatever is in the table.

The various options as per gh-3009 have not changed and should be unaffected. The only output change is the user/group columns as above.