feature request: annotate for blueocean where cache restore/save takes place

[rbtcollins]

Consider this executed pipeline. The caching step occurs after 'record pre-cache state' and before the parallel work immediately after. And the save step occurs at the end. But there is no visual marking for this, and as far as I can tell I can't manually work around it by adding stages or whatever - I could have a stage that only happens after the cache save, but the save time itself would still not be visible without dropping down to the underlying log.