Add THIRD_PARTY_LICENSES.csv file

Created by: tsenart

This file is generated with https://github.com/sourcegraph/license_scout which has a pending contribution upstream (https://github.com/chef/license_scout/pull/230).

Once that is merged upstream and a new Gem is released, I'll create a Docker image that we can use to run this periodically with little effort.

Fixes #8659