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/proxy.rb

@@ -0,0 +1,57 @@
+class Kamal::Configuration::Proxy
+  include Kamal::Configuration::Validation
+
+  DEFAULT_HTTP_PORT = 80
+  DEFAULT_HTTPS_PORT = 443
+  DEFAULT_IMAGE = "basecamp/kamal-proxy:latest"
+
+  delegate :argumentize, :optionize, to: Kamal::Utils
+
+  def initialize(config:)
+    @proxy_config = config.raw_config.proxy || {}
+    validate! proxy_config
+  end
+
+  def enabled?
+    !!proxy_config.fetch("enabled", false)
+  end
+
+  def hosts
+    if enabled?
+      proxy_config.fetch("hosts", [])
+    else
+      []
+    end
+  end
+
+  def image
+    proxy_config.fetch("image", DEFAULT_IMAGE)
+  end
+
+  def container_name
+    "kamal-proxy"
+  end
+
+  def publish_args
+    argumentize "--publish", [ "#{DEFAULT_HTTP_PORT}:#{DEFAULT_HTTP_PORT}", "#{DEFAULT_HTTPS_PORT}:#{DEFAULT_HTTPS_PORT}" ]
+  end
+
+  def deploy_options
+    {
+      host: proxy_config["host"],
+      "deploy-timeout": proxy_config["deploy_timeout"],
+      "drain-timeout": proxy_config["drain_timeout"],
+      "health-check-interval": proxy_config.dig("health_check", "interval"),
+      "health-check-timeout": proxy_config.dig("health_check", "timeout"),
+      "health-check-path": proxy_config.dig("health_check", "path"),
+      "target-timeout": proxy_config["response_timeout"]
+    }.compact
+  end
+
+  def deploy_command_args
+    optionize deploy_options
+  end
+
+  private
+    attr_accessor :proxy_config
+end