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/cli.rb

@@ -1,4 +1,5 @@
 module Kamal::Cli
+  class BootError < StandardError; end
   class HookError < StandardError; end
   class LockError < StandardError; end
 end