Implement the Logstash output

[cmacknz]

The same as https://github.com/elastic/elastic-agent-shipper/issues/10 and https://github.com/elastic/elastic-agent-shipper/issues/116, but for Logstash.

The agent supports a Logstash output today, so the shipper must implement a Logstash output to maintain feature parity with the outputs already available today.

Acceptance Criteria

• An integration test exists proving that the shipper can communicate with Logstash.
• An end to end test exists proving the agent can configure the shipper to communicate with Logstash.
• At least manual performance testing of the Logstash output has been performed.

[jlind23]

@leehinman any PR to provide here? Saw you made some progress on your fork.