Add item stac json file to list of assets when calling the "to_stac" method

[mike-gangl]

In order for the stac file to be staged out, it must be added as an asset to the item.json entry.

Currently this has to be done by the application package creator:

    # the future metadata file needs to be added to the STAC as well
    # will eventually be moved into the to_stac() function
    dataset.add_data_file(DataFile("metadata", output_path + '/' + path.name +'.json' ))

this is confusing because we're creating an asset for a file that does not exist, but will once we publish the STAC file. This should be handled by the unity stac libraries.