An EquivalenceClass is a set of Arc<dyn PhysicalExpr>s that are known
to have the same value for all tuples in a relation. These are generated by
equality predicates (e.g. a = b), typically equi-join conditions and
equality conditions in filters.
Calculates the union (in the sense of UnionExec) EquivalenceProperties
of the given EquivalenceProperties in eqps according to the given
output schema (which need not be the same with those of lhs and rhs
as details such as nullability may be different).