Add Cleanup to snapshot API

Cleanup is an optional method a snapshotter may implement.
Cleanup can be used to cleanup resources after a snapshot
has been removed. This function allows a snapshotter to defer
longer resource cleanup until after snapshot removals are
completed. Adding this to the API allows proxy snapshotters
to leverage this enhancement.

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

api/next.pb.txt

@@ -3446,6 +3446,16 @@ file {
       json_name: "inodes"
     }
   }
+  message_type {
+    name: "CleanupRequest"
+    field {
+      name: "snapshotter"
+      number: 1
+      label: LABEL_OPTIONAL
+      type: TYPE_STRING
+      json_name: "snapshotter"
+    }
+  }
   enum_type {
     name: "Kind"
     value {
@@ -3529,6 +3539,11 @@ file {
       input_type: ".containerd.services.snapshots.v1.UsageRequest"
       output_type: ".containerd.services.snapshots.v1.UsageResponse"
     }
+    method {
+      name: "Cleanup"
+      input_type: ".containerd.services.snapshots.v1.CleanupRequest"
+      output_type: ".google.protobuf.Empty"
+    }
   }
   options {
     go_package: "github.com/containerd/containerd/api/services/snapshots/v1;snapshots"