Auto merge of #9427 - nikkisquared:implement_main_fetch, r=jdm

Partial implementation of Main Fetch step

I started out with Step 11 of Main Fetch so I could test creating filtered responses, which broke my tests that compare the Fetch result to a message on a server. I realized that if I got the tests to work, I'd likely end up breaking them again with the next step of Main Fetch I added, so I went ahead and did as much of Main Fetch as I could figure out.

Some steps I'm sure I could implement, I just don't know how. Such as when the spec says to "wait for response", or how to implement a Runnable object (which iirc is what I need to use) to run everything after Step 8 in parallel.

The fetch tests are still not running correctly, but I sure it's because they're getting a filtered response which doesn't have the body of response. I'm not sure how to handle that, whether it means a change needed in the tests or in the Fetch code. Like always, I look forward to feedback on my work!

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