server.conf options for data tiering and index set templates

## Expected Behavior

I should be able to configure index settings in server.conf

## Current Behavior

Currently, as far as I can tell, Index Templates and Data Tiering are only configurable in the Graylog UI

## Possible Solution

Add config options in server.conf to control Index Templates and Data Tiering

## Context

We deploy graylog via chef cookbook, so configuration is done automatically via a templated server.conf. Generally, we steer clear of manual configuration of infrastructure.

## Your Environment

* Graylog Version: 6.2
* Java Version: n/a
* OpenSearch Version: 2.15
* MongoDB Version: 6.x
* Operating System: Ubuntu 22.04
* Browser version: n/a