Maintenance mode

Adds support for maintenance mode to Kamal.

There are two new commands:
- `kamal app maintenance` - puts the app in maintenance mode
- `kamal app live` - puts the app back in live mode

In maintenance mode, the kamal proxy will respond to requests with a
503 status code. It will use an error page built into kamal proxy.

You can use your own error page by setting `error_pages_path` in the
configuration. This will copy any 4xx.html or 5xx.html files from that
page to a volume mounted into the proxy container.

lib/kamal/configuration/proxy.rb

@@ -44,7 +44,8 @@ class Kamal::Configuration::Proxy
       "forward-headers": proxy_config.dig("forward_headers"),
       "tls-redirect": proxy_config.dig("ssl_redirect"),
       "log-request-header": proxy_config.dig("logging", "request_headers") || DEFAULT_LOG_REQUEST_HEADERS,
-      "log-response-header": proxy_config.dig("logging", "response_headers")
+      "log-response-header": proxy_config.dig("logging", "response_headers"),
+      "error-pages": error_pages
     }.compact
   end
 
@@ -52,6 +53,17 @@ class Kamal::Configuration::Proxy
     optionize ({ target: "#{target}:#{app_port}" }).merge(deploy_options), with: "="
   end
 
+  def stop_options(drain_timeout: nil, message: nil)
+    {
+      "drain-timeout": seconds_duration(drain_timeout),
+      message: message
+    }.compact
+  end
+
+  def stop_command_args(**options)
+    optionize stop_options(**options), with: "="
+  end
+
   def merge(other)
     self.class.new config: config, proxy_config: proxy_config.deep_merge(other.proxy_config)
   end
@@ -60,4 +72,8 @@ class Kamal::Configuration::Proxy
     def seconds_duration(value)
       value ? "#{value}s" : nil
     end
+
+    def error_pages
+      File.join config.proxy_error_pages_container_directory, config.version if config.error_pages_path
+    end
 end