[Link Event Damping] Add Abstract SelectEventHandler class to handle

link event damping timer events.

- Handler in the class will be used to handle the timer event of link
  event dampers.

HLD: sonic-net/SONiC#1071

syncd/SelectableEventHandler.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "swss/logger.h"
+#include "swss/sal.h"
+
+namespace syncd
+{
+
+    // This class implements handler for Selectable events.
+    class SelectableEventHandler {
+        public:
+
+            virtual ~SelectableEventHandler()
+            {
+              SWSS_LOG_ENTER();
+            }
+
+            virtual void handleSelectableEvent() = 0;
+
+        protected:
+
+            SelectableEventHandler()
+            {
+              SWSS_LOG_ENTER();
+            }
+    };
+
+}  // namespace syncd