Proposal: Add communication_period and impact_period

[Skander Chouchene]

A pull request was opened on 2025-03-25 to deprecate active_period and add communication_period (time in which the alert is communicated) and impact_period (time in which the disruption caused by the alert is actually happening).

So far for testing, VPE has supported the fields as a producer and MOTIS has supported the fields as a consumer.

https://github.com/google/transit/pull/546