add RegisterOrUpdateEventHook() function

caddy.go

@@ -1048,7 +1048,6 @@ func ConfigDigest(cdyfile Input) ([64]byte, error) {
 	if err != nil {
 		return [64]byte{}, err
 	}
-
 	sblocksJson, err := json.Marshal(sblocks)
 	if err != nil {
 		return [64]byte{}, err

plugins.go

@@ -271,6 +271,15 @@ func RegisterEventHook(name string, hook EventHook) {
 	}
 }
 
+// RegisterOrUpdateEventHook plugs in hook. All the hooks should register themselves
+// and they must have a name.
+func RegisterOrUpdateEventHook(name string, hook EventHook) {
+	if name == "" {
+		panic("event hook must have a name")
+	}
+	eventHooks.Store(name, hook)
+}
+
 // EmitEvent executes the different hooks passing the EventType as an
 // argument. This is a blocking function. Hook developers should
 // use 'go' keyword if they don't want to block Caddy.