Define a field other than `message` for the message summary in a messages table

## What?

It would be very useful to have a way to specify a field other than `message` as the message row shown when viewing a messages table widget.

## Why?

Currently one can customize the message summary (not the feature message summaries nor summary templates) using the `Format String` decorator. This is VERY useful to define custom summaries fairly easily. However, the only way to do this is to replace the content of the message field, which is destructive if you want to retain the contents of the original message field in this view. Being able to specify an arbitrary field to show as the summary would address this and allow more greater flexibility.

Let me know if this doesn't make sense or if you want to have a quick chat about this topic.

IMO this is a much more elegant way to define message summaries than messing with message summary templates.

## Your Environment

* Graylog Version: 5.2.1
* ~~Elasticsearch~~ OpenSearch Version: 2.8.0
* MongoDB Version: 6.0.6
* Operating System: Ubuntu 22.04 LTS
* Browser version: Google Chrome Version 114.0.5735.133 (Official Build) (arm64)