Cherry-pick #16156 to 7.6: Include document_id in decode_json_fields allowed fields

[faec]

Cherry-pick of PR #16156 to 7.6 branch. Original message:

What does this PR do?

This is a followup to #15859 which (among other things) added the document_id field to the decode_json_fields processor, but forgot to also add it to the list of allowed fields in the processor registry, so libbeat rejected configurations that used it. This PR adds it to the allowed fields list.

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