Allow 'Configure Index Set' to configure index settings for OpenSearch

## What?

Allow 'Configure Index Set' to configure index settings for OpenSearch

two specific examples shared with us are:

* `index.unassigned.node_left.delayed_timeout` - https://www.elastic.co/guide/en/elasticsearch/reference/current/delayed-allocation.html
* `index.search.slowlog.*` - https://opensearch.org/docs/latest/install-and-configure/configuring-opensearch/logs/#search-request-slow-logs
* NOTE: the above link talks specifically about the cluster level setting, but there may be use cases where one would only want to configure slow logs from some indices but not the entire cluster

Another possible addition to this is allowing the user to provide custom JSON that is merged into the index template that graylog creates/manages. While possibly risky or difficult to validate, this would allow the greatest flexibility.

## Why?

Some more mature and advanced users have needs to define things like `index.unassigned.node_left.delayed_timeout` (to prevent a shard reassignment storm if a node is temporarily offline) and `index.search.slowlog` to better understand slow queries.