Add schema for a data resource reference

[Zarquan]

A data resource place holder that can be defined in the metadata for an executable, and replaced by a real data resource when the user selects the data they want to use.
Basic properties such as the name, the IVOA data product type (from ObsCore)

resources:
  data:
    - name: input data
      ivoa:
        obscore:
          dataproduct_type: spectra

This acts as a hint for the UI to guide the user to select the right type of data.
It can also be referenced by compute resource volumes to specify the mount point.