Support the `timestamp` processor in packetbeat

**Describe the enhancement:**
The `timestamp` processor is missing from packetbeat (and auditbeat/metricbeat).

**Describe a specific use case for the enhancement or feature:**
In the logged events, I have an `event.start` and `event.end` time that both are less that the `@timestamp` field. I would like to make the `event.start` field the timestamp of the events.

Start by tracing how processors and event timestamps are handled in packetbeat, auditbeat, and metricbeat. Use the reported event.start, event.end, and @timestamp case as the acceptance example, and verify that the timestamp processor is supported consistently in the affected beats.

Written by the indexing model from the issue text.