add OnDisconnect() handlers.

handlers.go

@@ -61,6 +61,10 @@ func (rl *Relay) HandleWebsocket(w http.ResponseWriter, r *http.Request) {
 	)
 
 	kill := func() {
+		for _, ondisconnect := range rl.OnDisconnect {
+			ondisconnect(ctx)
+		}
+
 		ticker.Stop()
 		cancel()
 		if _, ok := rl.clients.Load(conn); ok {

relay.go

@@ -56,6 +56,7 @@ type Relay struct {
 	CountEvents               []func(ctx context.Context, filter nostr.Filter) (int64, error)
 	OnAuth                    []func(ctx context.Context, pubkey string)
 	OnConnect                 []func(ctx context.Context)
+	OnDisconnect              []func(ctx context.Context)
 	OnEventSaved              []func(ctx context.Context, event *nostr.Event)
 
 	// editing info will affect