opt: Add pruning rule for columns on the right side of Semi/Anti joins

[andy-kimball]

Consider this schema and query:

create table t1 (a int primary key, b int);
create table t2 (c int primary key, d int);
explain (opt,types) select * from t1 where exists(select * from t2 where c=a);

The optimized query uses a semi-join:

semi-join (merge)
 ├── columns: a:1(int!null) b:2(int)
 ├── left ordering: +1
 ├── right ordering: +3
 ├── scan t1
 │    └── columns: a:1(int!null) b:2(int)
 ├── scan t2
 │    └── columns: c:3(int!null) d:4(int)
 └── filters (true)

However, the d column can be pruned here, since it is not used by the join, and is not returned as an output column.

We should add a new prune_cols.opt rule that prunes unused columns on the right side of a Semi or Anti join (assuming the columns are in the PruneCols rule property set).