DRILL-7006: Add type conversion to row writers
Modifies the column metadata and writer abstractions to allow a type conversion "shim" to be specified as part of the schema, then inserted as part of the row set writer. Allows, say, setting an Int or Date from a string, parsing the string to obtain the proper data type to store in the vector.
Type conversion not yet supported in the result set loader: some additional complexity needs to be resolved.
Adds unit tests for this functionality. Refactors some existing tests to remove rough edges.
closes #1623
