[firebase_storage] Convert StorageReference to Url String

[awhitford]

It seem uneven that one can convert a fully qualified Url into a StorageReference using FirebaseStorage.getReferenceFromUrl, but then cannot easily get a fullUrl from a StorageReference.

One example use case is building a Firebase Image Cache where the Storage Reference URL (as a String) is a database primary key. Provided a StorageReference, convert it to a Url String, perform database lookup, if exists and coherent, serve from the cache, otherwise fetch from Storage and then populate the database cache.