CWLRequirementsItem

A single entry of a process's requirements list: any one of the requirement classes this register models — the standard CWL process requirements (DockerRequirement, ResourceRequirement, InitialWorkDirRequirement, ...) plus the cwltool-specific CUDARequirement extension. Kept as its own reusable, $ref-able union so downstream profiles can narrow the set of accepted requirement classes without depending on extensionPoints.