journalctl: add --output-fields= (#7181)

journalctl: add --output-fields= (#7181)

This option allows restricting the shown fields in the output modes that
would normally show all fields. It allows clients that are only
interested in a subset of the fields to access those more efficiently.
Also, it makes the resulting size of the output more predictable.

It has no effect on the various `short` output modes, because those
already only show a subset of the fields.