Add possibility to automatically stop processing if catched up

[breml]

When the dead_letter_input plugin is used to investigate failing log events in the dead letter queue, I normally use the commit_offsets option set to false and I start an additional instance of Logstash with a minimal config to make the content of the DLQ visible as suggested in https://www.elastic.co/guide/en/logstash/7.0/dead-letter-queues.html#processing-dlq-events.

The problem I have is, that with this config Logstash will never complete the task because the default behaviour of the dead_letter_input plugin is to poll the DLQ infinitly for new arriving events. Due to this I have to kill Logstash with Ctrl+c after some (hopefully long enough) period of time.
During the investigation phase it would be nice to have an option, which instructs the dead_letter_input plugin to stop processing when it has catched up to the end of the DLQ in a similar way as the stdin input plugin does it, when stdin is closed.
A possible implementation of this feature could be based on a duration, during which no new events are seen in the DLQ. This also would make it possible to post-process the output of Logstash with other tools (in our case with a pipe to jq ).