Add Prometheus AlarmManager support to Graylog's notifications

Add navitve Prometheus AlarmManager support to Graylog's notifications.

## What?
At present, there is a plugin on the Graylog marketplace which adds Prometheus AlarmManager support to Graylog: [/GDATASoftwareAG/Graylog-Plugin-AlertManager-Callback ](https://github.com/GDATASoftwareAG/Graylog-Plugin-AlertManager-Callback)

Unfortunately, it is reliant on legacy alarm callback handling.

As of Graylog 4.2.0, I see this warning when selecting legacy alarm callback in the Alarms tab.


It looks like as of now there is no integrated way for customers to have alarms send notifications up to Prometheus AlarmManager excepy by calling an external script, which is a bit miserable.

Given there is a plugin with code ready to go, it would be nice to add this functionality to the core Graylog product.

## Why?

We now provide prometheus compatible metric outputs. It seems likely that Graylog users will want to integrate prometheus into their stack. Prometheus AlarmManager is therefore a native choice for alerting that customers may wish to use.

The handling is already outlined in an existing plugin. We are deprecating the handling that plugin uses. Customers currently using Prometheus AlarmManager will have their setup broken by upgrading Graylog past 4.1.x

* Graylog Version:
4.2.0