Add firestore() method to api::QuerySnapshot (#3232)

Author: var-const

Firestore/core/src/firebase/firestore/api/document_change.h

@@ -17,6 +17,7 @@
 #ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_API_DOCUMENT_CHANGE_H_
 #define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_API_DOCUMENT_CHANGE_H_
 
+#include <memory>
 #include <utility>
 
 #include "Firestore/core/src/firebase/firestore/api/document_snapshot.h"
@@ -58,6 +59,10 @@ class DocumentChange {
     return new_index_;
   }
 
+  const std::shared_ptr<Firestore>& firestore() const {
+    return document_.firestore();
+  }
+
   /**
    * A sentinel return value for old_index() and new_index() indicating that
    * there's no relevant index to return because the document was newly added