`setLayerSettingForKey()` doesn't work on Overrides or DataOverrides
`setLayerSettingForKey()` on a layer works just fine, but using it on an Override/DataOverride throws an error.

According to the [documentation](https://developer.sketch.com/reference/api/#set-a-layer-setting), this should work.

## Reproduction (using Sketch DevTools)

1. Create a symbol with a text override;
2. Create a symbol instance and select it;
3. Set a Layer Setting to the override:
```js
const override = sketch.getSelectedDocument().selectedLayers.layers[0].overrides[0];
sketch.Settings.setLayerSettingforKey(override, 'key', 'value');
```

Throws `setValue:forKey:onObject:forPluginIdentifier: requires non-nil values for key, layer and pluginIdentifier`