Returns viewer assets for a dataset, along with a CloudFront signed
policy for accessing the asset files. Each asset includes an asset_url.
To access files, append the relative path and signing params:
{asset_url}{path}?Policy={policy}&Signature={signature}&Key-Pair-Id={key_pair_id}

Operates in one of two modes depending on package_id:

• package_id supplied — returns assets linked to that package
  (package-scoped assets).
• package_id omitted — returns assets in the dataset that have
  no package links (dataset-scoped assets).