Add kamal-proxy in experimental mode

The proxy can be enabled via the config:

```
proxy:
  enabled: true
  hosts:
    - 10.0.0.1
    - 10.0.0.2
```

This will enable the proxy and cause it to be run on the hosts listed
under `hosts`, after running `kamal proxy reboot`.

Enabling the proxy disables `kamal traefik` commands and replaces them
with `kamal proxy` ones. However only the marked hosts will run the
kamal-proxy container, the rest will run Traefik as before.

lib/kamal/configuration/role.rb

@@ -58,10 +58,18 @@ class Kamal::Configuration::Role
     default_labels.merge(traefik_labels).merge(custom_labels)
   end
 
+  def labels_for_proxy
+    default_labels.merge(custom_labels)
+  end
+
   def label_args
     argumentize "--label", labels
   end
 
+  def label_args_for_proxy
+    argumentize "--label", labels_for_proxy
+  end
+
   def logging_args
     logging.args
   end