Limit JSON extractor depth

## Expected Behavior

The JSON extractor could be used on deeply nested JSON messages to be parsed only up to a certain depth. Say, for a message like:

```
{"a": {"b": {"c": {"d": {"e": "f"}}}}, "g": "h"}
```

It could extract the following keys with a depth of 2:

```
a_b = {"c":{"d":{"e":"f"}}}
g = h
```

## Current Behavior

Currently the JSON extractor extracts all objects to the full depth. For the example above, we'd get

```
a_b_c_d_e = f
g = h
```

## Context

https://community.graylog.org/t/removing-key-from-pipeline-map/12037

I want to parse complex JSON messages that might be deeply nested without creating loads of keys from it. It's specially troublesome for messages that might use id-like keys which will cause the ES index to contain a large number of fields

## Your Environment

* Graylog Version: 3.1.0
* Elasticsearch Version: AWS 6.8
* MongoDB Version: 4.2.0