Consider adding a nonce to reports to prevent double counting

[johnivdel]

As documented in the PCM draft spec, https://privacycg.github.io/private-click-measurement/#receving-reports, adding a nonce to reports makes it possible to prevent double counting.

Attribution Reporting would similarly benefit from such a field, see https://github.com/WICG/conversion-measurement-api/issues/228.

@johnwilander, it would be great if we could align on a name for this potential field.

A few ideas:

• report_nonce (one issue is this could be confused with source_nonce for the blind signatures in #41 )
• report_id

[apasel422]

Following up on this to come to an agreement on a name for the field.

[apasel422]

CC @csharrison.