In preparation to support more policy engines in addition to the standard
Congress policy engine (e.g., ones based on SQL, influxDB, or Z3), we need
explicit typing of table columns in the data sources and the
data service engine. This spec proposes to support explicit typing, including
custom types, in an extensible, evolutionary, and backwards-compatible manner.
Change-Id: I389ff5fc8c7cdf9ca3b145c5eb8826f6e6cd28fa
58770c5037