Java projects overview

This issue is a meta-issue covering some projects within Logstash that are moving older code from Ruby to Java.

With major migrations like this, I like to sprinkle in some greenfield opportunities that we could include along the way, and will note them on each project. Remember, these opportunities are optional. I include them here because we have discussed wanting to do some of these things, and during these projects, we have an opportunity (since someone is deep in the code already) to make these improvements.

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(I will edit this issue and add links as issues are created)

Internals:

* Making pure-Java plugins possible
* [Java Plugin API.](https://github.com/elastic/logstash/issues/7986)
* Tooling for packaging and publishing Java plugins
* Documentation for how to write Java plugins
* Execution Engine

* Opportunity: Add debugger hooks to the pipeline execution.
* Metrics system
* Web API
* Opportunity: Security
* Event API with tighter restrictions on keys and values -- and type checking.
* Opportunity: Reduce Event methods to just set, get, delete, and serialization (remove to_hash, etc).

Plugins:

* Elasticsearch plugins (input, filter, output)
* Opportunity: Merge all Elasticsearch plugins into a single repo
* Opportunity: Split the Elasticsearch output plugin into separate plugins
* Grok
* Opportunity: Check alignment with Elasticsearch Ingest Node GrokProcessor
* Beats input (and output, which doesn't exist yet).
* Opportunity: Use a single repo for this, similar to the Elasticsearch opportunity
* HTTP input/output
* Opportunity: Live in the same repo