[uss_qualifier] Generate FeatureCheckTable at runtime when configured

[BenjaminPelletier]

This PR addresses #802 by adding the ability to generate a FeatureCheckTable for the geospatial feature comprehension test scenario at runtime. The test designer provides the URL of a Jsonnet file and, optionally, the URLs of one or more dict-like source files upon which the FeatureCheckTable contents will be based. The contents of the specified resources are retrieved and provided to the Jsonnet file as extVars, and then the Jsonnet file produces a FeatureCheckTable.

This approach is demonstrated by expanding the geospatial_comprehension.yaml CI test configuration to add another instance of the geospatial feature comprehension scenario, this time using a FeatureCheckTable generated at runtime. The incident_translator.jsonnet test_data is used to generate the FeatureCheckTable by reading a GeoJSON file (incidents.json) and creating a row for each Point or Polygon feature that isn't closed and isn't too stale.

The Jsonnet in this use case is basically a full, functional script, so users will likely benefit from tools that help with rapid iteration during development of this Jsonnet. So, a small generate_featurechecktable.py tool is provided to just generate the FeatureCheckTable based on a provided specification, and this tool is documented in the README file in the same folder.