Auto merge of #9753 - nikkisquared:async_fetch, r=jdm

Make Fetch Protocol Asynchronous

I'm working on making it possible to run Fetch Asynchronously, as required for some steps, such as Main Fetch. It looks like somebody has already laid some groundwork for that, with a AsyncFetchListener trait and two async fetch functions defined, which I'm building on top of.

So far, as a sort of proof of concept, I've written a test to asynchronously retrieve a fetch response, which uses a simple function to check if the fetch response is complete or not. I'd like to be checked if I'm on the right path, to see if I need to rework anything so far, and what my next step can be.

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