Teardown: Add a variant receiving context as parameter

[samber]

And add UnsubscribeWithContext(context.Context) method to ro.Subscription interface.

func MyOperator[T any]() func(Observable[T]) Observable[[]T] {
	return func(source Observable[T]) Observable[[]T] {
		return NewUnsafeObservableWithContext(func(subscriberCtx context.Context, destination Observer[[]T]) Teardown {
			sub := source.SubscribeWithContext(
				subscriberCtx,
				NewObserverWithContext(
					...
				),
			)

			return func(unsubscriberCtx context.Context) {
                                sub.UnsubscribeWithContext(unsubscriberCtx)
                         }
		})
	}
}

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