Add early encoding support to the Logstash output

[faec]

Experimental PR adding early encoding support for Logstash. ("Experimental" mostly because I didn't add or update any automated tests.)

Our main Beats benchmark pipeline doesn't support Logstash yet, so numbers here are from ad hoc tests ingesting randomly generated json events averaging 2KB in (original) size with default settings. However, the numbers are consistent with the more rigorous measurements for Elasticsearch early encoding.

Memory reduction ingesting with Filebeat using the filestream input and the Logstash output, relative to other recent versions:

Relative to main: 18% reduction
Relative to 8.13.2: 26% reduction
Relative to 8.12.2: 52% reduction

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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