Add disk_attachment labels

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#### Description

There is [a way](https://developer.hashicorp.com/packer/integrations/hashicorp/googlecompute/latest/components/builder/googlecompute#extra-disk-attachments) to build persistent disks, but no way to put labels on them

#### Use Case(s)

Labels help organize your resources

#### Potential configuration

```
disk_attachment {
volume_type = "pd-ssd"
volume_size = 60
keep_device = true
labels = {
env = "test"
project = "packer"
}
}
```

#### Potential References
https://cloud.google.com/sdk/gcloud/reference/compute/disks/add-labels