Add minimal ETW tracing to Kestrel for request correlation

[analogrelay]

We should add RequestStart and RequestStop ETW events to Kestrel so we can easily correlate Stress-related events within a single request. This will also be useful for performance monitoring of Kestrel and user web apps. We don't need detailed ETW, just a start/stop tracker. ETW handles the rest so that other ETW events get tagged with the right Activity ID

Tracking this here for now just to discuss this before we go to far. I'll make an issue in the Kestrel repo when we actually have a plan. Obviously whatever we do has to be cross-platform so it'll likely have to be ifdef/PlatformHelper gated for now.

/cc @davidfowl @DamianEdwards @halter73