refactor: stand-alone spawner service; remove proxy and nginx

[lihebi]

This is a major refactor, mostly around how runtimes are spawned. The new architecture is pictured below.

• The system now consists of 4 services: UI, API, Yjs, Spawner
• Notable, a new "spawner" service unifies the runtime-related pieces previously spread across api/proxy/yjs.

Change to development workflow:

• Now, nginx is removed. You no longer to go to http://localhost:8080 to see the app.
• Instead, the services now explicitly binds to host port (see compose.yml file for 4 ports bindings). Use http://localhost:3000 to access the app.

[senwang86]

Awesome change to simplify the development stack ! 🚀 🚀