[Spec] Spec Protected Audience aggregate reporting integration

[gtanzer]

Fill in the TODO in "report a private aggregation event": https://wicg.github.io/fenced-frame/#report-a-private-aggregation-event

This will entail changes to fenced frame config (adding more fields to "fenced frame reporting metadata") https://wicg.github.io/fenced-frame/#fencedframetype-fenced-frame-reporting-metadata and then hooking it into the config declarations in Protected Audience spec https://github.com/WICG/turtledove/issues/1158 (and Shared Storage selectURL spec if relevant).