Netflow does not extract nor store IPv6 addresses

## What?

Netflow does not extract nor store IPv6 addresses.

I'm unsure if this issue is related:
* https://github.com/Graylog2/graylog2-server/issues/6076

I'm also unsure if the default Netflow 9 field definitions only explicitly declaring `ip4_*` and `ipv4_*` but not any ipv6 counterparts matters or not.

Let me know if there are any questions.

## Why?

IPv6, while not as prevalent as IPv4, is growing in adoption and usage. Users and Customers have data sources and logs that include IPv6 addresses and would like these addresses to be treated as equal as IPv4 addresses.

## Your Environment

* Graylog Version: 6.x