GELF Output - feature to specify the record termination

## What?

In the GELF Output settings there is no option to set the record terminator - by default it uses NULL BYTE termination for each GELF message.

It would be useful to be able to be able to specify NEWLINE termination and potentially other character(s).

## Why?

The NULL BYTE termination doesn't work with the Google Chronicle/SecOps message forwarder - which expects JSON records to use NEWLINE termination. I expect other forwarding tools that could interoperate with Graylog might have different requirements and not support JSON with NULL BYTE termination.

## Your Environment

* Graylog Version: 6.3.8
* OpenSearch Version: 2.19.3
* MongoDB Version: 6.0
* Operating System: Rocky Linux 8
* Browser version: Chrome latest