Hide the true number of aggregatable reports

[csharrison]

This issue tracks the open question the aggregatable explainer: https://github.com/WICG/conversion-measurement-api/blob/main/AGGREGATE.md#hide-the-true-number-of-attribution-reports

To solve this, I believe we will need to have a mechanism that allows us to either randomize the true number of attribution reports, or make it a function of non-sensitive information (e.g. the # of unattributed trigger pings).

The presence or absence of an attribution report leaks some potentially sensitive cross-site data in the current design. Therefore, revealing the total count of reports to the reporting origin could leak something sensitive as well (imagine if the reporting origin only ever registered a conversion or impression for a single user).

To hide the true number of reports, we could:

- Unconditionally send a null report for every registered attribution trigger (thus making the count a function of only destination-side information)
- Add noise to the number of reports by having some clients randomly add noisy null reports. This technique would have to assume some threshold number of unattributed triggers to maintain privacy.