Refactor differ into separate package

Add differ options and package with interface.
Update optional values on diff interface to use options.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

api/next.pb.txt

@@ -1120,6 +1120,34 @@ file {
       type: TYPE_STRING
       json_name: "ref"
     }
+    field {
+      name: "labels"
+      number: 5
+      label: LABEL_REPEATED
+      type: TYPE_MESSAGE
+      type_name: ".containerd.services.diff.v1.DiffRequest.LabelsEntry"
+      json_name: "labels"
+    }
+    nested_type {
+      name: "LabelsEntry"
+      field {
+        name: "key"
+        number: 1
+        label: LABEL_OPTIONAL
+        type: TYPE_STRING
+        json_name: "key"
+      }
+      field {
+        name: "value"
+        number: 2
+        label: LABEL_OPTIONAL
+        type: TYPE_STRING
+        json_name: "value"
+      }
+      options {
+        map_entry: true
+      }
+    }
   }
   message_type {
     name: "DiffResponse"