ULA and external GUA IPv6 support on GCP

High Level Features:

1. VMs are dual-stack.

2. Subnets can have either GUA or ULA addressing along with IPv4.

3. Subnets are /64 and VMs get /96 address.

4. ULA addressing is useful for intra VPC traffic.

5. GUA addressing is useful if internet facing connection is required.

6. VMs can have multiple IPv6 NICs.