(output.elasticsearch) force user agent to be logstash

👋 howdy, team!

For the Logstash API outputs or at least for [its Elasticsearch output](https://www.elastic.co/guide/en/logstash/current/plugins-outputs-elasticsearch.html) would you kindly consider forcing its HTTP user agent to report as Logstash? "Manticore" still creeps through sometimes-almost-randomly which most team mates don't know how to interpret.

Example from Elastic Cloud proxy logging showing even recent versions affected by `user_agent: Manticore *`



Vs expected user agent formats `Logstash/*` like


TIA! 🙏

(Self-note: data uncertainty would be further exposed once [es#112845](https://github.com/elastic/elasticsearch/issues/112845) done, but concern would be reduced by [ls#16448](https://github.com/elastic/logstash/issues/16448) if implemented instead.)