Populate attribute that indicates whether `resource` spans were served from cache

[bcoe]

Problem Statement

It would be valuable for answering certain types of performance questions to be able to see whether resources displayed on the page are being served from cache.

Solution Brainstorm

Populate attribute on span based on deliveryType property (which I believe indicates whether or not the asset was returned from the browser cache?).

It would be even better if we could someone get the response headers for a resource, so that we could see whether or not it was served from a CDN.