[whatwg/fetch] Add a speculative flag to `Reqeust` as well as related processing (#881)

As discussed on https://github.com/w3c/resource-hints/issues/74 and on https://github.com/whatwg/html/pull/4115, this PR adds a speculative flag to `Request`, which makes sure the request:
* Survives navigations
* Is low priority
* Can be ignored by the UA (haven't added that part yet)
* Adds the related request headers

I intend (as part of https://github.com/whatwg/html/pull/4115) to then make sure prefetch requests are sent with this flag set.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/881

-- Commit Summary --

  * Add a speculative flag to Reqeust, and related processing

-- File Changes --

    M fetch.bs (31)

-- Patch Links --

https://github.com/whatwg/fetch/pull/881.patch
https://github.com/whatwg/fetch/pull/881.diff

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