Difficult to mutate nested args

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Issue details

I followed the ideas in https://github.com/pulumi/pulumi/issues/8584 to see how Java SDK stacks up for mutating nested properties of args. The idea is that users might want to write methods that abstracts some aspect of argument construction, and apply these methods to mutate something. Mutation is just what is easy with nested arguments. An immutable API is clunky and gives rise to helpers lenses, optics, which I don't think we have scope to support here.

The outcome of a quick investigation is positive: users can currently write functions that modify nested args. It just takes a little patience as it has to be done via builders and via the Output layer. For example:

public class MutableArgsClass extends Stack {
    public MutableArgsClass() {

        var deploymentArgsBuilder = DeploymentArgs.builder();

        // Cannot get DeploymentSpecArgs builder from DeploymentArgsBuilder. Can only set the args wholesale,
        // not modify them.
        deploymentArgsBuilder.spec(DeploymentSpecArgs.builder().replicas(2).build());
        var deploymentArgs = deploymentArgsBuilder.build();

        // We cat get nested args like .spec.replicas, but we cannot set them. Everything is also
        // wrapped with Output making it extra hard.
        Output<Integer> n = deploymentArgs.getSpec().apply(x -> x.getReplicas().applyValue(y -> y + 1));

        // Perhaps we can after all "modify" the nested replicas argument through some tricks?
        new Deployment("d", withExtraReplicas(deploymentArgs, 3));
    }

    public DeploymentArgs withExtraReplicas(DeploymentArgs args, int extraReplicas) {
        Output<DeploymentSpecArgs> deploymentSpecArgs = args.getSpec()
                .apply(spec ->
                    spec.getReplicas().applyValue(replicas ->
                        DeploymentSpecArgs.builder(spec)
                                .replicas(replicas + extraReplicas)
                                .build()));
        return DeploymentArgs.builder(args).spec(deploymentSpecArgs).build();
    }
}

So good news is that this is possible, bad news is that it's clunky. Any ideas how to simplify this are welcome.

Steps to reproduce

Expected:
Actual:

Start with the nested DeploymentArgs and DeploymentSpecArgs builder usage in the issue example, then review the Output-based mutation path and the related pulumi/pulumi#8584 discussion. Done would mean agreeing on and implementing a simpler supported way to mutate nested arguments, with validation that the example remains usable.