This directory should contain code that handles policies which rely on external data.
Each policy has one of three different formats:
In the EXTERNAL case, the policy value is (internally) represented by a JSON string that contains the URL of the external file, and a hash for verification. The code in this directory is responsible for the retrieval, verification and caching of the external data. Behavior that is specific to individual policies is defined via handlers in the external_data/handlers/ subdirectory.