Add support for retrieving background fetch response bodies
<rdar://problem/105770593>
Pull request: https://github.com/WebKit/WebKit/pull/10510
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/38771
Committed 261047@main (9800cc8170bb): <https://commits.webkit.org/261047@main> Reviewed commits have been landed. Closing PR #10510 and removing active labels.