Add "Aggregate Reporting" section

[jonasz]

Hi Michael,

As a follow up to the discussion during FLEDGE call on May 26 2021, I'm creating a pull req that extends FLEDGE with aggregate reporting.

Relevant issues: #177, #93, #164, #172.

Best regards,
Jonasz

[ALamraniAlaouiScibids]

Thank you @jonasz for this PR.
It is in fact critical for the adtech industry to have this API part of the original trials of FLEDGE.
Not having it would be very harmful for many businesses.

[michaelkleber]

@shivanigithub PTAL and see if this is in line with your reporting worklet thinking?
@MattMenke2 I don't think anything here should be a problem with information-passing expectations.

[jonasz]

Pullreq #273 added a reference to Private Aggregation API explainer which mentions FLEDGE, but doesn't go into much details.

I was wondering, what are the plans for working out how Private Aggregation API should be used in FLEDGE, could this pull request be a starting point? It seems to me it's still very relevant, modulo certain technical details that are now out of date (like referring to attributionsourcecontext which has been removed from ARA).

Let me know, at a high level, is this pull request is something that you have in mind as well? If so, I'd be happy to refresh it, and update the technical details.

Friendly ping @alexmturner @michaelkleber