Allow extra command args for on-work-request-completed

~> debusine on-work-request-completed "echo bla"
Error: "echo bla" does not exist or is not executable
3 ~> debusine on-work-request-completed echo bla
usage: debusine [-h] [--server SERVER] [--scope SCOPE] [--config-file CONFIG_FILE] [-s] [-d]
                {list-work-requests,show-work-request,create-work-request,manage-work-request,retry-work-request,provide-signature,create-workflow-template,create-workflow,create-artifact,import-debian-artifact,download-artifact,show-artifact,show-artifact-relations,create-asset,list-assets,on-work-request-completed,setup} ...
debusine: error: unrecognized arguments: bla
2 ~>

Would be great to give args to the executed command.

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