Heap dump target

See: https://www.openmicroscopy.org/community/viewtopic.php?f=5&t=6471

With these optional target blocks, it's possible to turn on heap-dumps-on-OOMs without modifying the config files. E.g.,
```
bin/omero admin start heap-dump
# or ... start heap-dump-tmp
```

If the `<target>` element is removed from around the `<option>` element, then heap-dumps will be activated by default (without passing "heap-dump").