Support Watch App Containers

[brentleyjones]

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.