[Spec] Additional bids and negative targeting feature (#796)

* Spec for negative targeting.

* Handling auction config for additional bids.

* progress

* Rename pending promise count to scoring dependency count.

* Finish additional bids, including reporting.

* Finish negative targeting.

* Address comments.

* Fix promise handling issue.

* Modify how addition bids promise is handled.

* Require interest group buyers.

* Address comments.

* Address Maks's comments.

* Change to [=parse an https origin=]

* Add a note: additional bid promise resolves after fetch finishes.

* Address comments.

* Check ig's ads being null.

* Updated IG cannot have both ads and additional bid key.

* fix build error.

* Negative IG cannot be updated.

* Address Dom's comments.

* Add non-normative, and link to adAuctionHeaders.

* Define Ad-Auction-Additional-Bid header.

* Apply suggestions from code review

Co-authored-by: Dominic Farolino <[email protected]>

* Apply suggestions from code review

Co-authored-by: Dominic Farolino <[email protected]>

* Address comments.

* Address remaining comments.

* Add link to a github issue for TODO.

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Co-authored-by: Qingxin Wu <[email protected]>
Co-authored-by: Dominic Farolino <[email protected]>