Want database model support for dual-stack network interfaces

[bnaecker]

Today, the database tables and model types for network interfaces support only one IP address family. The code itself is mostly agnostic to the IP version, but we've only used IPv4 so far. We need to add support for an address of either family, including dual-stack. Note that this refers specifically to the VPC-private IP addressing. External IP addresses are tracked separately.

• Add a nullable column / model field for an IPv6 address
• Make the existing IP address column / field specifically for IPv4 addresses, and make it nullable
• Add a check constraint that at least one of those fields is non-null