Refactor generatePredicate method from EnumerableNestedLoopJoin/EnumerableHashJoin/EnumerableBatchNestedLoopJoin into a single location

The method EnumerableNestedLoopJoin#predicate (that generates a Predicate Expression based on a RexNode condition) has been copied pasted as EnumerableBatchNestedLoopJoin#generatePredicate due to CALCITE-2979, and EnumerableHashJoin#generatePredicate due to CALCITE-2973.
The goal of this ticket is to refactor that method into a single location (e.g. EnumUtils) where it can be accessible by all 3 Enumerable Joins.