Pipeline substring function does not support selecting an output from list

## Expected Behavior

Most other functions in the pipeline supports selecting a output from the list, but not substring. There should be an option to do so.

## Current Behavior

Currently the substring function only supports direct string.

## Possible Solution

Supporting output list for substring function.

## Context

I'm trying to shorten/truncate log messages ($message.message) that are longer than 100 characters, but I'm currently unable to do so. So adding the ability to select an output from list would be useful.
I was originally trying to substring under Alerts - Custom HTTP Notification (which would make my life much easier rather than doing a whole bunch of work to create a pipeline with rules) because I'm sending the log to other sources.

## Your Environment

* Graylog Version: 6.1.5+e3ae3ce
* Operating System: Debian 12