[whatwg/fetch] Trailer support in the API (#981)

I still think this is something we should do as it's part of HTTP and with newer iterations of H/2 it's a feature that's a lot easier to make use of due to overall improved infrastructure.

My current thinking is that building this on top of #980 and #607 (as you can have multiple trailer header lists per request/response) is the way to go. `FetchObserver` could have a `sendTrailer(Headers trailer)` and `ontrailer` event handler or some such. Details probably best sorted once there's more firm implementer interest.

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