Adds secrets field to ListSnapshotsRequest (#372)

There is no way to directly pass credentials to ListSnapshots call.
This change will add a field required to pass credentials by the
caller.

Author: Hakan Memisoglu

csi.proto

@@ -1009,6 +1009,11 @@ message ListSnapshotsRequest {
   // and will not block if the snapshot is being processed after
   // it is cut.
   string snapshot_id = 4;
+
+  // Secrets required by plugin to complete ListSnapshot request.
+  // This field is OPTIONAL. Refer to the `Secrets Requirements`
+  // section on how to use this field.
+  map<string, string> secrets = 5 [(csi_secret) = true];
 }
 
 message ListSnapshotsResponse {