Add rotateonstartup configuration for output.file

[gitck]

In our setup the apm server is running locally on the same host as the monitored application and is configured output.file. We do not want to loose recorded data because of the file rotation, only because the system is restarted several times in succession.
Therefore we would appreciate the implementation of the feature rotateonstartup as it already exists for the logfiles (https://www.elastic.co/guide/en/apm/server/current/configuration-logging.html#_logging_files_rotateonstartup).
See also https://discuss.elastic.co/t/is-it-possible-to-disable-rotation-of-file-completely/235813 for the previous discussion.

[axw]

@gitck thanks for opening the issue! This applies to all of the Beats too, so I'll transfer to the beats repo.

I'm not sure when we'll get to this. If you feel like opening a PR to move things along faster, please take a look at the contributing guide: https://www.elastic.co/guide/en/beats/devguide/current/beats-contributing.html

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)