ocrd workspace: add action to mets:agent, too

[bertsky]

All our processors now add provenance information about the run to the METS via mets:agent. This is useful for diagnosis and reconstruction of the workflow later-on.

However, the METS actions done by ocrd workspace are not included here. So when there is a rename-group or bulk-add or remove-group, we cannot see that directly.

IMO ocrd.cli.workspace should do a workspace.mets.add_agent similar to ocrd.processor.run_processor. We could add the action and options under notes kwarg analogously.