Add requiredSellerCapabilities check when processing auctionReportBuyers

[alexmturner]

Recreates patcg-individual-drafts/private-aggregation-api#134 now that the monkey patches have been incorporated into this repo.

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[alexmturner]

@morlovich, could you PTAL? Just realized I never got back to you on your earlier PR; so, recreated it here.

To be honest, I still don't quite understand the new check here -- the explainer seems to indicate that such buyers would be entirely excluded from the auction (ref). But I defer to you on what's most aligned with the implementation.

[morlovich]

So the motivation was that, despite these buyers being excluded from the auction, our implementation currently can still report interest group counts for them (if they had that capability but not some other reported capability). I don't remember the context of the spec change, though, and at this very moment my brain is too loaded with another task to work out whether the spec part is needed. My intent of sending the spec change was to nail down if our behavior needed to be changed, though.