Allow adding additional NICs to instance creation

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#### Description

Allow adding additional NICs when Packer is provisioning the virtual machine used to create the image.

#### Use Case(s)

Some software I am trying to install in an image requires having multiple nics (attached to different subnets) on the machine, and without them, it causes problems. I was wondering if it is possible to have multiple NICs attached to the instance that packer creates so the software installs properly. It might be too niche of a use case, but the current software causes problems when installed with a single nic, so that when you try to boot the image that packer produces, it cannot be reached.

#### Potential configuration

Perhaps something like
```
source "googlecompute" "windows-ssh-example" {
project_id = "MY_PROJECT"
...

network_interface {
...
}

network_interface {
...
}
}
```

#### Potential References

This is supported in terraform when creating a [google_compute_instance](https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_instance#network_interface), but it is not clear to me if this would even be possible in packer.