Add protected destructor (#37215)

c++

@drfloob @yashykt

Closes #37215

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37215 from d7d1cd:virtual_destructor 861ad17eda
PiperOrigin-RevId: 652578386

include/grpc/event_engine/extensible.h

@@ -60,6 +60,9 @@ class Extensible {
   /// if (endpoint != nullptr) endpoint->Process();
   ///
   virtual void* QueryExtension(absl::string_view /*id*/) { return nullptr; }
+
+ protected:
+  ~Extensible() = default;
 };
 
 }  // namespace experimental