Case insentive regexp search via `case_insensitive` parameter

## What?
Case insensitive regular expression search

## Why?
Both Elasticsearch >= 7.10 and Opensearch >= 1.x support the `case_insensitive` regexp parameter. By default it is disabled, but one can manually set it to true. However, Graylog lacks the means to set `case_insensitive` to true, preventing case-insensitive regular expression searches.

## Your Environment

* Graylog Version: 5.2.7
* OpenSearch Version: 1.3.15