Save Index Set in Content Pack

## What?
Ability to save the stream/index_set in a content pack.

Saw this [post](https://community.graylog.org/t/index-set-and-stream-in-the-content-pack-is-possible/5707) so i asked [again](https://community.graylog.org/t/index-set-in-content-packs/13854) and it is still not supported. So creating this ticket as instructed.

## Why?
1. Backup purposes (the settings can get pretty large/complex...)
2. Easy import to multiple graylog instances (we running 4 - 3 production - 1 testing)

## Your Environment
```
CentOS Linux release 7.7.1908 (Core) 3.10.0-1062.12.1.el7.x86_64
graylog-3.2-repository-1-1.noarch
graylog-integrations-plugins-3.2.1-1.noarch
graylog-server-3.2.1-1.noarch
graylog-plugin-auth-sso-3.2.1-1.noarch
mongodb-org-tools-4.0.16-1.el7.x86_64
mongodb-org-server-4.0.16-1.el7.x86_64
mongodb-org-shell-4.0.16-1.el7.x86_64
mongodb-org-4.0.16-1.el7.x86_64
mongodb-org-mongos-4.0.16-1.el7.x86_64
elasticsearch-6.8.6-1.noarch
```

Hope that covers everything.
As always, thank you for the support!

-Dave