Filter based aggregatable deduplication

[Nan Lin]

Hi everyone,

As proposed in issue 664, we plan to support filter based aggregatable deduplication, similar to the deduplication mechanism in event-level reports.

We're considering removing the existing aggregatable_deduplication_key field in Attribution-Reporting-Register-Trigger header which was introduced in M108. Note that this is a backward incompatible change and the aggregatable deduplication key set in the old way (aggregatable_deduplication_key) will be silently dropped.

Please let us know if there's any concern with this change. 

Thanks,
Nan