Let the Expert provision instances with libraries and plugins already installed

From the Expert demo. When someone asks for something that needs third party nodes, and the Expert also has to create the instance to build it on, there's no way for that instance to come up with the nodes already there. The Expert creates a bare instance and then writes flows that reference nodes nobody installed.

At creation time the Expert can pick the instance type, stack, template and blueprint, but there's nothing that says "and install these packages too".

Would be good if the Expert could pass along the packages a plan needs and get back an instance that's ready for them. Might be worth leaning on what templates and blueprints already do here rather than adding a separate path, but that's open.