Permit user programs that decide on stack outputs in a Future

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Issue details

Current signature Pulumi.run assumes that the program will immediately register the stack outputs:

        Pulumi.run(ctx -> {
            ctx.export("a", Output.of(1));
        });

What if the program does not know what the set of stack outputs is going to be until some CompletableFuture? The awkward workaround right now is to force (join) the Future inside the Pulumi program:

        CompletableFuture<String> xname;
        Pulumi.run(ctx -> {
            var x = xname.join();
            ctx.export(x, Output.of(1));
        });

It would be nice to allow deferring that without blocking a thread. Perhaps Pulumi.run(Function<Context,CompletableFuture<Void>>) overload to let the user program signal to the framework that it has some work to do.

I believe underlying framework supports this just fine, but we do not expose this capability to the user at the moment.

Not sure how useful this is in practice, but logging for posterity.

Affected area/feature

Start by tracing the existing Pulumi.run API and the framework's current handling of CompletableFuture-based work. Clarify the overload design, then verify that deferred stack outputs can be registered without blocking and add coverage for the asynchronous completion behavior.