Introduction of FencedFrameConfig and deprecation of opaque URNs for fenced frames

[Kevin Lee]

Previously, we supported opaque URNs in fenced frames to render the ad creative; this support will be deprecated starting M114 Stable. We'll be removing the `src` attribute on fenced frame elements, and we are introducing a fenced frame config object (FencedFrameConfig) to navigate the fenced frame to an ad. Testing is available starting from M112. 

When you pass in the flag resolveToConfig into the runAdAuction() call, the auction will return a fenced frame config object, that you set as the config attribute for the ad space’s fenced frame. The config object contains a url property that is opaque. Additionally, you will be able to pass the embedder context into the fenced frame config which becomes available in a shared storage worklet later. 

If the flag is not provided or is set to false, an opaque URN will be returned which can only be rendered in an iframe. 

To learn more about the fenced frame config, see the article on the Chrome blog, and the explainer.