The JSON Extractor do not expand nested JSON *array* objects

The JSON Extractor do not expand nested JSON *array* objects

## Expected Behavior

Splunk use multivalue fields (mv* SPL)
Graylog shoud do the same (vertical storage) or store a list/tuple objects (horizontal storage)

## Current Behavior

Grayog store the nested array JSON object in a separate field (with parent name), without expansion.

## Possible Solution

Splunk use multivalue fields (mv* SPL)
Graylog shoud do the same (vertical storage) or store a list/tuple object (horizontal storage)

## Steps to Reproduce (for bugs)

Office 365 Management Activity logs contain a nested JSON *array* objects AffectedItems when
* Workload = Exchange
* Operation = SoftDelete

Sample json with a nested JSON array object:

{"AffectedItems": [
{Attachments=image001.jpg (1234b); image002.jpg (1234b); image003.jpg (1234b), Id=Rg, InternetMessageId=, ParentFolder={Id=Lg, Path=\Inbox}, Subject=Quotation}, {Attachments=image003.jpg (1234b); image004.jpg (1234b), Id=Rg, InternetMessageId=, ParentFolder={Id=Lg, Path=\Inbox}, Subject=Test}, {Id=RA, InternetMessageId=, ParentFolder={Id=Lg, Path=\Inbox}, Subject=Notification}, {Id=Rg, InternetMessageId=, ParentFolder={Id=Lg, Path=\Inbox}, Subject=Documents}, {Attachments=image001.jpg (1234b); image002.jpg (1234b); image003.jpg (1234b), Id=Rg, InternetMessageId=, ParentFolder={Id=Lg, Path=\Inbox}, Subject=Works}]}

## Context

Can not use graylog for Office365 Management Activity log.

## Your Environment

* Graylog Version: v3.0.2+1686930
* Elasticsearch Version: 6.7.2
* MongoDB Version: 4.0.9
* Operating System: CentOS-7.6.1810
* Browser version: N/A