[STORY] Write `shell_cmd` task that can be initiated by build YAML

[tclose]

Description

Following the success of the approach used for arcana.tasks.bids.bids_app, where BIDS apps can be included in the YAML build recipes it would be good to do the same thing for generic shell commands with a similar function at arcana.tasks.common.shell_cmd

Acceptance Criteria

• [x] XNAT CS pipelines can be generated for generic shell commands
• [x] No additional Python code is required
• [x] Input descriptions used to generated XNAT CS command JSON can be reused to created shell input spec via YAML pointers

[tclose]

This has been implemented by Australian-Imaging-Service#139