Add Support for IPv6

[razorsedge]

Describe the Feature

Support IPv6.

Expected Behavior

IPv6 CIDRs should be allocated to the subnets. Auto-assign IPv6 address should be enableable.

Use Case

We need to assign IPv6 addresses to our VPC resources. cloudposse/terraform-aws-vpc already supports IPv6 so we just need to be able to assign to the subnets.

Describe Ideal Solution

Allow for passing in ipv6_cidr_block and either assume that the addresses should be auto-assigned or provide an argument to toggle IPv6 auto-assignment and default to enabled if ipv6_cidr_block is present.