Spec: add glue for Private Aggregation's per-context contribution limits

[dmcardle]

Explainer: patcg-individual-drafts/private-aggregation-api#146

Spec change: patcg-individual-drafts/private-aggregation-api#164

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[dmcardle]

@alexmturner PTAL!

This PR tries to integrate with patcg-individual-drafts/private-aggregation-api#164. The goal is to enable auctions and interest groups to (optionally) configure maxContributions.

I guess we'll want to merge the Private Aggregation PR first because this PR references [=pre-specified report parameters/requested max contributions=].

[dmcardle]

I guess we'll want to merge the Private Aggregation PR first because this PR references [=pre-specified report parameters/requested max contributions=].

Or maybe not. I used <var ignore> to silence the error for now.

[alexmturner]

Thanks! Per patcg-individual-drafts/private-aggregation-api#164 (comment), we might want to postpone this spec change until we have support for deterministic reports for Protected Audience sellers (and do all those changes in one go).