Consider allowing intra-source filtering to stop generating aggregatable reports

[johnivdel]

Currently, the Attribution-Reporting-Filters header allows a reporting origin to prevent the browser from generating reports based on trigger time signals and trigger time signals.

This is not sufficient in cases where a reporting origin might want to stop triggering after some criteria is met. For example, let's say the reporting origin only wanted to generate aggregatable reports for the first 10 triggers for a source.

A idea is to add a native filtering key similar to source_type which includes the current number of triggers attributed to the source. However it is possible there are similar features which could be achieved through a more robust mechanism.