Publication needs to include a reportedBefore field

[mcb30]

To prevent abuse, contact identifiers are valid only until the seed value is made public. This is already stated in the Seed Report section ("An Advertiser MUST NOT transmit any Contact Identifiers generated by a Generator instance once the Seed Report corresponding to that Generator’s Seed Value has been sent").

Observers need to know a time after which the seed value has been made public, since they should check for matching contact identifiers received only prior to this time.

The Publication currently conveys this information imperfectly via the publishedAt field, which loses information for seed values reported in previous publications.

Could be fixed by including a reportedBefore field within the Notification.