[Spec] Add trustedBiddingSignalsSlotSizeMode (#954)

* Add optional slot-size parameters to trustedBiddingSignalsURL requests

This is a slightly modified version of the proposal in issue 869

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Typo

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Switch to component auction.

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* [spec] Add trustedBiddingSignalsSlotSizeMode

And related auctionConfig field.

This corresponds to the explainer changes in #928.

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* Fix compile issues.

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* add an extra line to separate </div> from steps

* Partial response to comments.

* Partial changes

* More partial responses

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* Fix build?

* Fix merge

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Response to Dominic

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double->string

* Clean-up ad size conversion

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