ENH: Add IntervalIndex.is_overlapping
This is needed in the `get_indexer` implementation for the new `IntervalIndex` behavior, as an overlapping `IntervalIndex` may return non-unique indices for a given query.  Note that this equivalent to other indexes checking `is_unique` as part of `get_indexer`, which does not suffice for `IntervalIndex` for the aforementioned reason.  Behavior and API design would be comparable to `is_non_overlapping_monotonic`.

My plan is to implement this within the `IntervalTree` to take advantage of some cached sorting that it performs, so it would not be implemented for `IntervalArray` (we don't build an `IntervalTree` there, as `IntervalArray` is mutable, and doesn't need indexing methods).  This _could_ be implemented for `IntervalArray` if need be but might not be quite as performant.