Disallow selecting fields that cannot be aggregated when adding an alert aggregation

## What?

When adding an aggregation to an alert, it appears you can select any field, even fields that will fail to aggregate. It would be useful for graylog to disallow this.

## Why?

I mistakenly ran into an aggregation error when creating an alert aggregation and I just wanted it to count the number of messages. I naively selected the message field and saved the alert.


A graylog system alert was shown for `SEARCH_ERROR: Aggregation search failed`.

It is my understand that graylog already knows what fields can or cannot be aggregated because in the message table (for example) it disallows you from doing top values on the `message` field. This could help users avoid pitfalls.

## Your Environment

* Graylog Version: 5.1.3
* OpenSearch Version: 2.8.0
* MongoDB Version: 6.0.7
* Operating System: Ubuntu Server 22.04 LTS
* Browser version: Google Chrome Version 114.0.5735.198 (Official Build) (arm64)

Let me know if there are any questions!