[firebase_ui_storage] implement StorageImage widget

[lesnitsky]

We want to have an easy way to display images uploaded to firebase_storage, thus firebase_ui_storage should have a StorageImage.

Things to consider:

• Are we going to use CachedNetworkImage? Does it support HTTP cache-related headers?
• Do we use CachedNetworkImage directly or allow configuring ImageProviders?
• Do we provide our own StorageImageProvider?