Add OpenCowReader interface method to dynamic partition control

Test: treehugger
Change-Id: I393fc36372f9e979da1a6c7e49c7782a7372a5b9
diff --git a/common/dynamic_partition_control_interface.h b/common/dynamic_partition_control_interface.h
index c8a2274..a2ebfc6 100644
--- a/common/dynamic_partition_control_interface.h
+++ b/common/dynamic_partition_control_interface.h
@@ -154,6 +154,10 @@
       const std::string& unsuffixed_partition_name,
       const std::optional<std::string>&,
       bool is_append = false) = 0;
+  virtual FileDescriptorPtr OpenCowReader(
+      const std::string& unsuffixed_partition_name,
+      const std::optional<std::string>&,
+      bool is_append = false) = 0;
 
   // Create virtual block devices for all partitions.
   virtual bool MapAllPartitions() = 0;