Util to simplify execution of basic pivot search

## What?

Create a utility service that simplifies execution of basic pivot searches in the backend. It would accept the variable parts of the search (query, pivot field name) and handle the details of building the search, extracting the values from the result rows, and handling errors.

## Why?

- When coding a search for the first time, it is difficult to know which classes to use and how
- There are multiple instances of these searches in the code base. They are very similar to each other:
`EventDefinitionAggregator`
`InputDiagnosticService`
`EventAssetRiskAggregator`

## Your Environment

* Graylog Version: 6.2