Possibility to add priority levels on event definitions/alerts

## What?
Possibility to add priority levels on event definitions/alerts

## Why?
Today Graylog only has three levels of priority on alerts (to my knowledge), and many tools/solutions have in start four levels, low, medium, high and critical, just to have a more accurate classification.

Regardless of that, IMHO it will be much better if Graylog could allow us to add more priority levels and either change your designations (names).

## Your Environment

* Graylog Version: Graylog 4.3.3+86369d3
* Elasticsearch Version: elasticsearch-oss-7.10.2-1
* MongoDB Version: mongodb-org-server-4.2.18-1
* Operating System: CentOS Linux release 7.9.2009
* Browser version: Chrome 105.0.5195.52