new v3.0 Event API for timestamp initialization and coercion

This follows up on the idea in #4293

Some plugins might need to re-inject a timestamp and currently the `Event` timestamp [initialization and coercion](https://github.com/elastic/logstash/blob/c2ea4a7a6eedee46fb7d169773b150cc44c78713/logstash-core-event/lib/logstash/event.rb#L250-L266) is not exposed.

As an example, the json filter would really benefit this since it is actually reimplementing the timestamp initialization code, see https://github.com/logstash-plugins/logstash-filter-json/pull/18

At this point I am not sure if we should add a method in the `Event` or in `Timestamp` for this but either way we should make the timestamp initialization a public API and DRY the Event and the json filter.