Consider adding a way to bind a value, computed based on other observable values

As proposed by the @yousefak007 in https://github.com/s0nerik/context_plus/issues/12#issuecomment-2308998016, https://github.com/s0nerik/context_plus/issues/12#issuecomment-2309013971 and https://github.com/s0nerik/context_plus/issues/12#issuecomment-2311033969, this can probably be achieved with introduction of `Ref.bindProxy()` method with a specially-crafted type of `BuildContext` suitable for triggering the value re-computation without rebuilding the `context` upon a notification `.watch()`'ed from within the `create` callback. Proposed API: ```dart T Ref.bindProxy( context, T Function(BuildContext context, T? previous) create, { void Function(T value)? dispose, Object? key, }); ``` Another option could be to add a `Ref.maybeOf(context)` method and just provide the special version of a `BuildContext` to the existing `Ref.bind()` and `Ref.bindLazy()` methods: ```dart T Ref.bind( context, T Function(BuildContext context) create, { void Function(T value)? dispose, Object? key, }); ``` ```dart void Ref.bindLazy( context, T Function(BuildContext context) create, { void Function(T value)? dispose, Object? key, }); ```