Have providers extend each-others. Then deprecate ProviderBase

[rrousselGit]

It would be significantly helpful if the core providers subclassed each-others:

class Provider<T> {}

class FutureProvider<T> extends Provider<AsyncValue<T>> {}
typedef AsyncProvider<T> = FutureProvider<T>;
class StreamProvider<T> extends AsyncProvider<T> {}


class NotifierProvider<Notifier, T> extends Provider<T> {}
class AsyncNotifierProvider<Notifier, T> extends NotifierProvider<Notifier, AsyncValue<T>> {}
class StreamNotifierProvider<Notifier, T> extends AsyncNotifierProvider<Notifier, T> {}

We could then deprecate ProviderBase in favour of Provider itself.

The benefit of such change is that it enables folks to easily add extension methods to providers/ref, while having access to .future/.notifier when it makes sense.

This avoids having to introduce interfaces for "Providers with a .notifier or .future property"