pub.dev Example tab should link to example app source

Right now pub.dev's example section doesn't really help the user get to the example app's source code.

A "Example app source code" link pointing to https://github.com/FirebaseExtended/flutterfire/tree/master/packages/firebase_auth/firebase_auth/example would help get the user pointed in the right direction. We might not need to link to the GitHub version of README.md since it's already displayed.

This isn't a pub issue, but we probably shouldn't include a pub badge in the example app's README (it is confusing because example apps aren't independently versioned).