libbeat/publisher: debugging elastic agent publisher event log entries is unreasonably onerous

When debugging publication issues, it may be necessary to examine events in the logs/data/events for the actual event that is being published. Currently, this event is rendered into the long message field as text making it unnecessarily difficult to extract by standard tooling (e.g. `jq`). This happens in the [`debugPrintProcessor`](https://github.com/elastic/beats/blob/8b25d5b14bb12c02bac944f44f15b35a151a71cf/libbeat/publisher/processing/processors.go#L191-L216).

Start in libbeat/publisher/processing/processors.go at debugPrintProcessor, the location identified in the issue. Trace how the published event is placed in the log entry and make the event separately extractable by standard JSON tooling such as jq; verify the resulting log structure with the relevant publisher processing tests.

Written by the indexing model from the issue text.