Add mappers for box proto to client

plugins/commands/serve/mappers.rb

@@ -160,6 +160,7 @@ end
 # NOTE: Always directly load mappers so they are automatically registered and
 #       available. Using autoloading behavior will result in them being unaavailable
 #       until explicitly requested by name
+require Vagrant.source_root.join("plugins/commands/serve/mappers/box.rb").to_s
 require Vagrant.source_root.join("plugins/commands/serve/mappers/capabilities.rb").to_s
 require Vagrant.source_root.join("plugins/commands/serve/mappers/command.rb").to_s
 require Vagrant.source_root.join("plugins/commands/serve/mappers/direct.rb").to_s

plugins/commands/serve/mappers/box.rb

@@ -0,0 +1,20 @@
+module VagrantPlugins
+  module CommandServe
+    class Mappers
+      # Build a box client from a proto instance
+      class BoxFromProto < Mapper
+        def initialize
+          inputs = [].tap do |i|
+            i << Input.new(type: SDK::Args::Box)
+            i << Input.new(type: Broker)
+          end
+          super(inputs: inputs, output: Client::Box, func: method(:converter))
+        end
+
+        def converter(proto, broker)
+          Client::Box.load(proto, broker: broker)
+        end
+      end
+    end
+  end
+end