[Logs Overview] Detect changes for document groups without a message

[weltenwort]

:notebook: Summary

Some types of log documents don't have a message but consist purely of structured fields. In order to give the user an overview of those we could replace the categorization aggregation with a terms or significant terms analysis and do a change point detection for the buckets produced by those. The results could then be shown in the same data grid as the categories.

• :link: related to: https://github.com/elastic/kibana/issues/190848

:heavy_check_mark: Acceptance criteria

This task includes significant amounts of research and experimentation. Therefore the acceptance criteria will change over time.

• Log documents without a message field are shown in the data grid.
• They are derived from well-known, suitable categorical fields. (room for a lot of creativity here 🧑‍🎨)
• Change point detection is performed for the buckets formed that way too.
• Instead of their pattern a reasonable representation of the key/value pair is shown.

[elasticmachine]

Pinging @elastic/obs-ux-logs-team (Team:obs-ux-logs)