[Question] Using an IPV4_IPV6 stack type with compute instances

##### SUMMARY

I would like to create a compute instance that uses an existing dual stack subnet, and I would like to be able to ssh into the machine using the IPV6 address. I've tried simply adding the existing subnet to the machine's `network_interfaces` using the subnet's `selfLink`, but when I do that, only the IPV4 address is listed on the machine. The [`access_config`](https://docs.ansible.com/ansible/latest/collections/google/cloud/gcp_compute_instance_module.html#parameter-network_interfaces/access_configs) options seem rather limited, and I haven't figured out how to create a compute instance with an IPV4_IPV6 stack type with them. Is this supported? If so, do you have examples of how to do this?

##### ISSUE TYPE
- Question

##### COMPONENT NAME

gcp_compute_instance

##### ADDITIONAL INFORMATION

Start with the gcp_compute_instance module and its network_interfaces/access_configs documentation, then investigate how an existing dual-stack subnet is represented. Determine whether IPV4_IPV6 stack configuration is supported and whether an example can be provided; done means a confirmed configuration or a clearly identified implementation gap.

Written by the indexing model from the issue text.