Implement `allow_leading_wildcard_searches` and `allow_highlighting` as runtime configurations

## What?

By default, the server configuration does not permit search queries with leading wildcard operators. (e.g., `message:*ello`) Users can set `allow_leading_wildcard_searches = true` in the server configuration file to enable leading wildcard searches.

**UPDATE:** We could do the same for the `allow_highlighting` option. (see: https://github.com/Graylog2/graylog2-server/issues/7050)

It would be great if Graylog admins could change the setting at runtime via the web interface.

## Why?

Changing the leading wildcard search option requires restarting all Graylog servers in the cluster to activate it. That makes the option hard to change when the cluster is already running. Not having a configuration setting in the user interface makes it harder for users to discover the leading wildcard search feature.

## Your Environment

* Graylog Version: 4.3.0