Storage Image should have a configurable image provider

[lesnitsky]

StorageImage should have a configurable image provider, to allow local caching, similar to https://pub.dev/packages/firebase_cached_image.

It would make sense to configure a top-level image provider, so FirebaseUIStorageConfiguration will need an additional property defined and taken into account.

cc @russellwheatley

[github-actions]

Hello 👋, to help manage issues we automatically close stale issues.

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