override the redundancy based on the resizer

[poonai]

  let mut children = children
        .with_redundancy(3) // Start with 3 actors
        .with_heartbeat_tick(Duration::from_secs(5)); // Do heartbeat each 5 seconds

    #[cfg(feature = "scaling")]
    {
        children = children.with_resizer(
            OptimalSizeExploringResizer::default()
                .with_lower_bound(1) // A minimal acceptable size of group
                .with_upper_bound(UpperBound::Limit(2)) 
                .with_upscale_strategy(UpscaleStrategy::MailboxSizeThreshold(3)) // Scale up when a half of actors have more than 3 messages
                .with_upscale_rate(0.1) // Increase the size of group on 10%, if necessary to scale up
                .with_downscale_rate(0.2), // Decrease the size of group on 20%, if too many free actors
        );
    }

The above code will panic at https://github.com/bastion-rs/bastion/blob/master/src/bastion/src/resizer.rs#L254 because redundancy is 3 is higher than the upper bound.

This PR overwrites the minimum number actor will be spawned based on the recent argument pass. It can be via a resizer or redundancy.

Still, we need to catch the places like lower bound can't be higher than the upper bound. Wondering how we can put such sanity checks ergonomically.

[Relrin]

The fix for scaling could rely on the special call that will be called right after the value being set in the with_lower_bound/with_upper_bound method. It could be called something like align_bounds

But in general, this code looks ok to me.

[vertexclique]

looks good. thanks.