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.rb

@@ -10,7 +10,7 @@ class Kamal::Configuration
   delegate :argumentize, :optionize, to: Kamal::Utils
 
   attr_reader :destination, :raw_config, :secrets
-  attr_reader :accessories, :aliases, :boot, :builder, :env, :healthcheck, :logging, :traefik, :servers, :ssh, :sshkit, :registry
+  attr_reader :accessories, :aliases :boot, :builder, :env, :healthcheck, :logging, :proxy, :traefik, :servers, :ssh, :sshkit, :registry
 
   include Validation
 
@@ -60,6 +60,7 @@ class Kamal::Configuration
 
     @healthcheck = Healthcheck.new(healthcheck_config: @raw_config.healthcheck)
     @logging = Logging.new(logging_config: @raw_config.logging)
+    @proxy = Proxy.new(config: self)
     @traefik = Traefik.new(config: self)
     @ssh = Ssh.new(config: self)
     @sshkit = Sshkit.new(config: self)
@@ -143,6 +144,10 @@ class Kamal::Configuration
     traefik_roles.flat_map(&:hosts).uniq
   end
 
+  def proxy_hosts
+    proxy.hosts
+  end
+
   def repository
     [ registry.server, image ].compact.join("/")
   end