Automatically Call DisposeAsync

It's unsafe to rely on a user calling [`DisposeAsync`](https://github.com/Azure/azure-functions-sql-extension/blob/a2a8da1bc448321f1adfe64c7c7ffdc54f6c3238/src/SqlAsyncEnumerable.cs#L77) once they're done using the enumerator exposed by `SqlAsyncEnumerable`. If they do not call it, the `SqlConnection` and `SqlDataReader` will remain open, leaking resources. Ideally, we should find a way to have this method be called automatically once the user's function exits.