Cross-Advertiser Aggregation Queries

[bmcase]

We've discussed the functionality of cross-advertiser aggregation at length in the past (it was a functionality in proposals IPA, PAM, and Hybrid), and filing this issue to track adding it to Level 2 of Attribution.

Motivating Use Cases:

1. supporting ML optimization queries
2. providing measurement to small advertisers who would not have enough signal to be measured individually
3. providing ad-techs ways of measuring how their ads are performing across different breakdowns

Design Sketch:

1. intermediaries (ad-techs) can have DP budgets tracked on the device.
2. budget is available for use by intermediaries through the conversion site quota (in Big Bird)
3. the aggregation service then supports aggregating reports requested by the same intermediary