add outcome/status_code to resource timing spans

[devcorpio]

Context

Until recently, performance ResourceTiming entries were not exposing the response status code of any loaded resource.

Because of that, the RUM agent sets the outcome of every resource timing span to unknown. Since it's not possible to know whether a resource has loaded successfully.

Fortunately, since version 109, Chromium browsers have started to expose the aforementioned status code.

Opportunity

Due to that fact, we will be able to set the outcome (success, failure) of the resource timing span based on that status code, which will give our users much more visibility of what's going on with their web application resources.