Slack Notification - Make Custom Message searchable

## What?

From Customer request: https://github.com/Graylog2/support/issues/295
[gz#2381](https://graylog7416.zendesk.com/agent/tickets/2381)

Want to send alerts notifications to Slack in order to easily identify relevant events, but have a problem regarding the format in which they are received.

The custom message (which usually includes relevant backlog information, such as source IP addresses, ports, event details, etc.) is received in a format that uses the highlight box with the selected configuration color

Even if the “Include Title” checkbox is not selected. The drawback of the highlight box is that you can not use the search function of the Slack platform in order to search/filter relevant information, because of the custom message highlight formatting.

Intended Purpose:
Easily identify relevant events in Slack

Expected Outcome:
It would be very useful if you could allow the “custom message” to be sent without any highlight box. More precisely the “custom message” should be formatted like "event definition title" (which is in a normal message format for Slack platform), when the “Include Title” checkbox is not selected or you could add an extra checkbox to fulfil the format problem for “custom message".

## Why?

Easily identify relevant events in Slack. Highlight box does not allow the search function in Slack.

## Your Environment

* Graylog Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version: