feat: chat desktop backend (#23005)

Implement the backend for the desktop feature for agents.

- Adds a new `/api/experimental/chats/$id/desktop` endpoint to coderd
which exposes a VNC stream from a
[portabledesktop](https://github.com/coder/portabledesktop) process
running inside the workspace
- Adds a new `spawn_computer_use_agent` tool to chatd, which spawns a
subagent that has access to the `computer` tool which lets it interact
with the `portabledesktop` process running inside the workspace
- Adds the plumbing to make the above possible

There's a follow up frontend PR here:
https://github.com/coder/coder/pull/23006

coderd/coderd.go

@@ -1179,6 +1179,7 @@ func New(options *Options) *API {
 				r.Use(httpmw.ExtractChatParam(options.Database))
 				r.Get("/", api.getChat)
 				r.Get("/git/watch", api.watchChatGit)
+				r.Get("/desktop", api.watchChatDesktop)
 				r.Post("/archive", api.archiveChat)
 				r.Post("/unarchive", api.unarchiveChat)
 				r.Get("/messages", api.getChatMessages)