Publish export command and others as "client commands" available to Zookeeper

[franknoirot]

Companion to https://github.com/KittyCAD/text-to-cad/issues/4210. Export and other commands are available to ZDS right away, but if a user asks Zookeeper "export this model as a STEP" it will:

1. Receive and process all the current project contents
2. Connect to its own engine via websocket
3. Execute that model, then export it to STEP and save it as a conversation attachment
4. Return the STEP file as a part of its JSON-encoded reasoning messages

I'm prototyping if ZDS and other clients could be able to publish capabilities to Zookeeper using a update_client_command_schema with its command schema, then react to client_command_request messages from it by executing the command and replying with a client_command_response.

Export should be the first example, hidden behind a feature flag, to be published by ZDS, unless it turns out to be easier to just have all the commands get published 🙃.