Change the inputDatastorePath() to const

Change-Id: Ia3892798846bf75972cf345bc8236a7cbd3749eb
diff --git a/src/proxy_datastore.hpp b/src/proxy_datastore.hpp
index 20a877d..befde09 100644
--- a/src/proxy_datastore.hpp
+++ b/src/proxy_datastore.hpp
@@ -35,7 +35,7 @@
     [[nodiscard]] DatastoreAccess::Tree execute();
     void cancel();
 
-    std::optional<std::string> inputDatastorePath();
+    std::optional<std::string> inputDatastorePath() const;
 
     [[nodiscard]] std::shared_ptr<Schema> schema() const;