Add the ability to perform case insensitive searches

## What?

Search case sensitivity depends on the field’s mapping (field type + analyzer).
Most of the time it results in case sensitive searches.

It would be nice to be able to do case insensitive search queries (without modifying the field's mapping).
Especially OpenSearch supports such searches with the case_insensitive parameter:
- https://docs.opensearch.org/docs/latest/query-dsl/term/regexp/
- https://docs.opensearch.org/docs/latest/query-dsl/term/term/

Maybe we could add a flag in the regex syntax, for example:
```
user:/admin.*/i
```
Or:
```
user:/(?i)admin.*/
```

I know that for stream rules's regex we can add this flag (?i).
It would be really nice to have this for the search query.

## Why?

It would increase the search capacities and ease the job of analysts.