storage: support object custom metadata key deletes

**Is your feature request related to a problem? Please describe.**
This came up in https://github.com/GoogleCloudPlatform/golang-samples/issues/2066

**Describe the solution you'd like**
The GCS API allows for specific object custom metadata keys to be deleted, but is not possible in Go Storage library.

To delete a specific key an example JSON structure must be sent:
```
{
"metadata": {
"key-to-delete": null
}
}
```

Currently the Go Storage client only allows deleting all metadata keys.