Add tags to VolumeInfo

This patch adds an optional field "tags" to "VolumeInfo". The "tags"
field is a "map[string]string" and can contain additional attributes
related to a volume.

Author: akutz

csi.proto

@@ -247,6 +247,11 @@ message VolumeInfo {
   // REQUIRED. The identity information will be used by the CO in
   // subsequent calls to refer to the provisioned volume.
   VolumeHandle handle = 2;
+
+  // Tags captures additional attributes related to the volume. This
+  // field is OPTIONAL. This field is NOT REQUIRED when submitting
+  // a VolumeInfo object to ValidateVolumeCapabilities.
+  map<string, string> tags = 3;
 }
 
 // VolumeHandle objects are generated by Plugin implementations to