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/database/migrations/000436_add_chat_mode.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE chats DROP COLUMN mode;
+DROP TYPE IF EXISTS chat_mode;

coderd/database/migrations/000436_add_chat_mode.up.sql

@@ -0,0 +1,3 @@
+CREATE TYPE chat_mode AS ENUM ('computer_use');
+
+ALTER TABLE chats ADD COLUMN mode chat_mode;