Document building and debugging logstash-output-opensearch

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Is your feature request related to a problem? Please describe.

Developers are asking how to build and debug logstash-output-opensearch.

Describe the solution you'd like

A new section in DEVELOPER_GUIDE called "Building and Debugging".

Additional context

I wrote up steps to building in #186 (comment) against AWS. The developer guide should have instructions for both a local instance of OpenSearch and an Amazon managed service.

For debug logging I believe https://forum.opensearch.org/t/enable-debug-logging-on-logstash/12194 or if you run logstash from the command line, run with ./bin/logstash —debug.