Move type definitions out of propolis-client

There are several swaths of types, relating to instance specs and the serial console, defined in the propolis-client lib today. Some of those are then re-exported in that same library from the code generated by progrenitor, based of the openapi json defined by propolis-server. While the handmade client code in propolis-client served a purpose early-on, as the progenitor-based bits were still being hammered out, we should clean that up so that its little more than the progenitor definition, plus whatever small helpers we deem necessary.

Types exposed by the openapi-visible portion of propolis-server should go in some sort of crate or module, used only by propolis-server (and perhaps the mock-server), rather than living in propolis-client. This will cut down on the dependencies required by propolis-client consumers as well.

Start by inventorying the instance-spec and serial-console types in the propolis-client library, including the progenitor-generated re-exports, then compare them with the openapi-visible types in propolis-server. Determine an appropriate crate or module for server and mock-server use. Done means propolis-client retains only the progenitor definition and necessary helpers, while consumers no longer need its extra dependencies.