Extend metadata provider to pull up predicates through Union

Follow-up of HIVE-13068.

Currently, when we pull up predicates through Union operation with Calcite metadata provider, we just create a single disjunction of pulled up predicates.

E.g. Assume operators I1, I2, I3 with predicates P1, P2, P3 that can be pulled up through them, respectively.
For an operation Union (I1, I2, I3) we infer a new predicate Pu, such that Pu = P1 OR P2 OR P3.
While this is correct, we miss some chances for simplification e.g. if there are common factors in P1, P2, and P3. Further, this inference differs slightly from the way that the metadata provider pulls up predicates for other operators (a list of predicates), thus breaking some assumptions and missing some optimization opportunities.

Ex. ql/src/test/results/clientpositive/input26.q.out

explain
select * from (
  select * from (select * from srcpart a where a.ds = '2008-04-08' and a.hr = '11' order by a.key limit 5)pa
    union all
  select * from (select * from srcpart b where b.ds = '2008-04-08' and b.hr = '14' limit 5)pb
)subq