cmpopts: add EquateApproxDuration

#155 added the ability to provide a margin for comparing time.Time, but some projects have needed to define their own custom comparers to handle margins for duration comparisons. It seems like it occurs enough to potentially be nice to add a new `EquateApproxDuration` opt.

Some examples:
- [crossplane](https://github.com/crossplane/crossplane/blob/f018aac0689c4dea80096490ee5d712b6c123566/internal/controller/ops/cronoperation/reconciler_test.go#L41-L53)
- [vegeta](https://github.com/tsenart/vegeta/blob/cf5811269046c672a604b1eb352204d30f16ae4a/lib/metrics_test.go#L88-L113)
- [gotest.tools](https://github.com/gotestyourself/gotest.tools/blob/0b81523ff268a1f1b0baf4a5da67e42fbb86880b/assert/opt/opt.go#L14-L29)