Add snapshotter key to snapshot events

Consumers of snapshot events require the snapshotter
field to refer to the related snapshot.

Signed-off-by: Derek McGowan <derek@mcg.dev>

api/next.pb.txt

@@ -400,6 +400,13 @@ file {
       type: TYPE_STRING
       json_name: "parent"
     }
+    field {
+      name: "snapshotter"
+      number: 5
+      label: LABEL_OPTIONAL
+      type: TYPE_STRING
+      json_name: "snapshotter"
+    }
   }
   message_type {
     name: "SnapshotCommit"
@@ -417,6 +424,13 @@ file {
       type: TYPE_STRING
       json_name: "name"
     }
+    field {
+      name: "snapshotter"
+      number: 5
+      label: LABEL_OPTIONAL
+      type: TYPE_STRING
+      json_name: "snapshotter"
+    }
   }
   message_type {
     name: "SnapshotRemove"
@@ -427,6 +441,13 @@ file {
       type: TYPE_STRING
       json_name: "key"
     }
+    field {
+      name: "snapshotter"
+      number: 5
+      label: LABEL_OPTIONAL
+      type: TYPE_STRING
+      json_name: "snapshotter"
+    }
   }
   options {
     go_package: "github.com/containerd/containerd/api/events;events"