This commit introduces a new operator for ValuesScan, earlier we
generated `UNION ALL` for cases where VALUES lists passed are all
constants, but now a new Operator CLogicalConstTable with an array of
const tuples will be generated

Once the plan is generated by ORCA, it will be translated to valuesscan
node in GPDB.

This enhancement helps significantly in improving the total run time for the queries
involving values scan in ORCA with const values.
Signed-off-by: NEkta Khanna <ekhanna@pivotal.io>