Consider batching reports for the same source in the same reporting window

[johnivdel]

Currently, if a clicked source generates 3 reports in the same reporting window, the browser will issue 3 separate post requests to the reporting endpoint in a random order.

To save bandwidth and pre-join these reports for reporting endpoints, we could consider modifying the report body to contain an array of reports.

Because these are already join-able server-side this shouldn't have any privacy implications. This will add a small amount of complexity to the report body however.

[apasel422]

This could be something like:

{
  "source_event_id": "<x>",
  "source_type": "navigation",
  "triggers": [
    {"trigger_data": "<z1>"},
    {"trigger_data": "<z2>"},
    // ...
  ]
}