Support Watch App Containers

In Xcode you can create watch apps in one of two ways: hosted in an iOS app, or as a standalone watch app.

Currently rules_apple only supports the first version, via `ios_application` and the `watch_application` attribute. To support the second one we a new target that will create the iOS application stub with `com.apple.product-type.application.watchapp2-container` product type. Similar to `ios_imessage_application` a stub binary (and launch storyboard) are bundled along with the watch app itself.

Compare the existing ios_imessage_application implementation with ios_application and its watch_application attribute. Trace how the stub binary and launch storyboard are bundled, then determine the entry point for a standalone watch app target. Done means rules_apple supports the standalone product type and bundles the required stub components.

Written by the indexing model from the issue text.