ExtensionKit App Extensions not supported

App extensions like [Background Assets](https://developer.apple.com/documentation/backgroundassets) are of type `EXAppExtension`, instead of `NSExtension`, and have some different build settings. An `EXAppExtension` is of product type `com.apple.product-type.extensionkit-extension` and installed into the `Extensions` directory instead of `PlugIns`.

Trying to configure a Background Asset downloader extension with current Bazel rules will run into validation errors, since `PlugIns` expect an `NSExtension` dictionary in Info.plist, but the extension point `com.apple.background-asset-downloader-extension` disallows that property.

For now, we can work around this simply by moving the `.appex` bundle from `PlugIns` to `Extensions`.

Start by reproducing the validation errors for a Background Assets downloader extension and inspect the Info.plist expectations for the .appex bundle. Compare the current PlugIns placement with the requested Extensions placement; done means EXAppExtension bundles use the correct product type, build settings, installation directory, and plist validation.

Written by the indexing model from the issue text.