_simulate endpoint for Logstash

Ingest pipelines can be tested using the **_simulate** endpoint. But Logstash pipelines cannot.

To test a Logstash pipeline one must perform a "dry run" using test input.

A **_simulate** endpoint for Logstash that behaves the same way as for ingest pipelines would be very handy. It would also be a step toward more standardized (and easier to use) functionality.

Start by comparing the existing ingest-pipeline _simulate endpoint with the Logstash pipeline dry-run flow using test input. Define the endpoint behavior and integration points needed for Logstash pipelines, then verify that it provides equivalent simulation behavior without requiring a normal run.

Written by the indexing model from the issue text.