Add field grouping mechanism

## What?

In various places, users would like to access "internal" fields (most of which are currently prefixed by `gl2_`) because they are of legitimate interest. But sometimes those fields get in the way because they might not be easily understandable to less technical users.
Some of the places are:
* field list in the sidebar
* dropdown in parameter value list
* message details
* aggregation builder

## Why?
E.g. selecting `gl2_event_type` in a parameter value dropdown is currently not possible, because "internal" fields are filtered out there. However, it's not really internal, it's just prefixed to avoid clashes with other user-generated fields.
Wholesale displaying all fields everywhere might be very noisy, but we don't have a way of expressing this in the UI.

## Your Environment

* Graylog Version: 4.1
* Elasticsearch Version: n/a
* MongoDB Version: n/a
* Operating System: n/a
* Browser version: n/a