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/commands/app.rb

@@ -30,6 +30,24 @@ class Kamal::Commands::App < Kamal::Commands::Base
       role.cmd
   end
 
+  def run_for_proxy(hostname: nil)
+    docker :run,
+      "--detach",
+      "--restart unless-stopped",
+      "--name", container_name,
+      *([ "--hostname", hostname ] if hostname),
+      "-e", "KAMAL_CONTAINER_NAME=\"#{container_name}\"",
+      "-e", "KAMAL_VERSION=\"#{config.version}\"",
+      *role.env_args(host),
+      *role.logging_args,
+      *config.volume_args,
+      *role.asset_volume_args,
+      *role.label_args_for_proxy,
+      *role.option_args,
+      config.absolute_image,
+      role.cmd
+  end
+
   def start
     docker :start, container_name
   end