Add communication_period and impact_period to GTFS Service Alerts

[skalexch]

Describe the problem

In the GTFS Realtime Alert spec, active_period is defined as: Time when the alert should be shown to the user. If missing, the alert will be shown as long as it appears in the feed. If multiple ranges are given, the alert will be shown during all of them.

However, there still is an ambiguity of interpretation of the active_period field; whether it is intended for the period of showing the alert or for the period of the related service disruption itself.

Use cases

We need to distinguish:

1. The period when the alert is shown -> Communication period of the alert
2. The period when the alert is actually in effect -> Impact period of the alert

Proposed solution

We propose the addition of two fields, communication_period and impact_period, to disambiguate active_period.

The detailed proposal is found here: communication_period and impact_period proposal

Additional information

More information regarding the extent of this issue can be found in PR#482