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.

test/integration/integration_test.rb

@@ -31,7 +31,7 @@ class IntegrationTest < ActiveSupport::TestCase
         succeeded = system("cd test/integration && #{command}")
       end
 
-      raise "Command `#{command}` failed with error code `#{$?}`" if !succeeded && raise_on_error
+      raise "Command `#{command}` failed with error code `#{$?}`, and output:\n#{result}" if !succeeded && raise_on_error
       result
     end