Possibility to add infra_ip6 to devices

[vidarstokke]

Is your feature request related to a problem? Please describe.
We use the field "infra_ip" for devices to generate lo0 IP-address. We also need to be able to set a similar field for IPv6.

Describe the solution you'd like
We want a additional field to be available for devices. For instance called "infra_ip6". This field should be of type IPv6 address. The field should of course also be available as a setting for the device in templates as "infra_ip6".

Describe alternatives you've considered
We've considered automatically generating IPv6 address from the IPv4 address, but this does not work for us.

Additional context
The attached image shows a snippet from the documentation and marks the field that we want added as an IPv6 field.