spec: Add efficient volume cloning support

With the new CSI snapshot feature it's now possible to create volumes
from a snapshots.  This may lead to users abusing it for unintended
functionality, such as having a golden image to source new volumes.

Most storage backends support efficient volume cloning, so it would be
benefitial to expose this functionality in the CSI spec.

By supporting cloning we avoid users creating a volume, and then a
snapshot of that volume, just so it can be used as the source for new
volumes.

The VolumeContentSource, that was added by the snapshots feature, will
be used for this feature.  New VolumeSource message is added to the
VolumeContentSource.

csi.proto

@@ -253,8 +253,16 @@ message VolumeContentSource {
     string id = 1;
   }
 
+  message VolumeSource {
+    // Contains identity information for the existing source volume.
+    // This field is REQUIRED. Plugins reporting CLONE_VOLUME
+    // capability MUST support creating a volume from another volume.
+    string id = 1;
+  }
+
   oneof type {
     SnapshotSource snapshot = 1;
+    VolumeSource volume = 2;
   }
 }
 
@@ -778,6 +786,11 @@ message ControllerServiceCapability {
       // with the snapshot_id as the filter to query whether the
       // uploading process is complete or not.
       LIST_SNAPSHOTS = 6;
+      // Plugins supporting volume cloning at the storage level MAY
+      // report this capability. The source volume must be managed by
+      // the same plugin. Not all volume sources and parameters
+      // combinations may work.
+      CLONE_VOLUME = 7;
     }
 
     Type type = 1;