Add a plugin for determining disk usage.

This plugin utilises the work in gh-2919 to provide a summary of disk usage by object type:
```
bin/omero usage Image:1,2,3  --report
Using session b011dfc4-2d21-4898-b72b-0a366891414a (user-0@localhost:4064). Idle timeout: 10.0 min. Current group: pg-0
Total disk usage: 153629 bytes
 component    | size  
--------------+-------
 Job          | 59124 
 Annotation   | 8     
 FilesetEntry | 90432 
 Thumbnail    | 4065  
(4 rows)
```
This is very much work in progress in both the name, `usage` is not that good, the object arguments and the output styles, more for discussion than review at this stage.

/cc @mtbc @joshmoore @sbesson

--depends-on #2919
That makes good sense. (I don't know if there is some refactoring card somewhere upon which your method ought now be noted, or if it will probably be noticed anyway in later work.)