Generate other/missing bucket in aggregations

## What?

As a user, I would want to be able to include buckets for:

- documents which have values which fell out of the value limit for the pivot (Probably can be implemented easily only in a limited way)
- documents which are missing the field being pivoted on (Addressed by : https://github.com/Graylog2/graylog2-server/pull/12958)

These are currently not included.

## Why?

As a user it is often interesting to see how the values of the current result relate to the rest of the bucket and if they are quantitatively representative of the overall bucket.

## Your Environment

* Graylog Version:
* Elasticsearch Version:
* MongoDB Version:
* Operating System:
* Browser version: