Add resource timing spans with events to DocumentLoad plugin

[draffensperger]

I would suggest that we use the Resource Timing API to get spans for all of the resource timing events that occur before the end of the load event in the browser as part of the DocumentLoad plugin. Each span should include any network events (DNS resolution, SSL handshake, request start, etc.) as TimedEvents on the span when present.

We did this in OpenCensus Web, see this trace screenshot and this code link

@obecny what would you think about adding this feature?

[obecny]

please assign this to me