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/broken_deploy_test.rb

@@ -27,6 +27,5 @@ class BrokenDeployTest < IntegrationTest
       assert_match /First web container is unhealthy on vm[12], not booting any other roles/, output
       assert_match "First web container is unhealthy, not booting workers on vm3", output
       assert_match "nginx: [emerg] unexpected end of file, expecting \";\" or \"}\" in /etc/nginx/conf.d/default.conf:2", output
-      assert_match 'ERROR {"Status":"unhealthy","FailingStreak":0,"Log":[]}', output
     end
 end