GCS Data Source

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#### Description

It would be nice if a GCS data source existed that would allow one to download GCS objects during packer builds. These could then be referenced in `file` provisioners for upload to the build instance.

#### Use Case(s)

This would be useful when using non-GCP builders that need access to private data stored in a GCS bucket.

#### Potential configuration

```hcl
data "googlestorage" "my_file" {
bucket = "a-bucket"
object = "path/to/a.file"
}

...

build {
...
provisioner "file" {
sources = data.googlestorage.my_file.local_path
destination = "/tmp/a.file"
}
}
```

---

If there isn't already an accepted solution for this, and there is also interest, I am happy to work on a PR for this feature.