Add missing annotations map to Descriptor for gRPC transfer

Add the annotations map to the gRPC Descriptor message.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Brandon Lum <lumjjb@gmail.com>
Signed-off-by: Harshal Patil <harshal.patil@in.ibm.com>

api/next.pb.txt

@@ -1764,6 +1764,34 @@ file {
       type: TYPE_INT64
       json_name: "size"
     }
+    field {
+      name: "annotations"
+      number: 5
+      label: LABEL_REPEATED
+      type: TYPE_MESSAGE
+      type_name: ".containerd.types.Descriptor.AnnotationsEntry"
+      json_name: "annotations"
+    }
+    nested_type {
+      name: "AnnotationsEntry"
+      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
+      }
+    }
   }
   options {
     go_package: "github.com/containerd/containerd/api/types;types"