DRILL-5258: Access mock data definition from SQL
Extends the mock data source to allow using the full power of the mock
data source from an SQL query by referencing the JSON definition
file. See JIRA and package-info for details.
Adds a boolean data generator and a varying-length string generator.
Adds “mock” table stats for use in the planner.
Revisions based on code review comments
close #752
