Add support for pushing joins through a union

Given a query structure such as:

inner join
union all
a
b
union all
c
d

where a,b,c,d are access nodes to a source that supports joins and there is a constant column projected by each union branch and that column is used in the join predicates, then we should be able examine

union all
inner join
a
c
inner join
a
d
...

to determine if any of the joins will no longer be necessary.

This is a possible pattern in a multi-source or partitioning scenario.

In some scenarios there may not be an actual projected literal, which could be accommodated if a foreign key could be marked as partitioned.