Add include_message to the list of Filestream parsers

[belimawr]

Proposed commit message

Add include_message to the list of Filestream parsers

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.

## Disruptive User Impact
## Author's Checklist

How to test this PR locally

1. Make sure you have https://github.com/elastic/docs cloned at the same level as Beats
2. Export $GIT_HOME as the folder where you have cloned docs and beats
3. Build the docs/start the web server:

   $GIT_HOME/docs/build_docs --respect_edit_url_overrides --doc $GIT_HOME/beats/filebeat/docs/index.asciidoc --resource=$GIT_HOME/beats/x-pack/filebeat/docs --chunk 1 --open
   

4. Navigate to http://localhost:8000/guide/filebeat-input-filestream.html#_parsers
5. Ensure include_message is in the list of parsers.
   ## Related issues
   ## Use cases
   ## Screenshots
   ## Logs

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @belimawr? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[rdner]

Just a heads up for other reviewers: the parser had been documented in https://www.elastic.co/guide/en/beats/filebeat/8.13/filebeat-input-filestream.html#_include_message before this PR. It was just missing on the list.