Instance create form: clarify immediate creates vs. create on submit

We could add/are maybe planning to add a button that lets you create a floating IP right there, and that would cover it. The biggest issue for me is making clear which operations actually create stuff (e.g., floating IP, SSH key) and which are queuing things up to be created as part of instance create (e.g., disks). I wonder if the visual treatment could convey that somehow, or we can just put it in an info box at the top of the forms.

Originally posted by @david-crespo in https://github.com/oxidecomputer/console/issues/2252#issuecomment-2125205930

It's not clear what the behavior is when you add resources like disks and SSH keys on instance create, and we want the user to be able to click around and try things and know whether they're making real changes to the system.

Start at the instance create form and trace how floating IPs, SSH keys, and disks are handled when users add them. Done means the interface clearly distinguishes resources created immediately from those queued for instance creation, allowing users to understand the effect before submitting.