fix: ws open handler hook

Author: bosi95

src/bee.ts

@@ -1384,6 +1384,10 @@ export class Bee {
     ws.onopen = () => {
       startPing()
       flushQueue()
+
+      if (handler.onOpen) {
+        handler.onOpen(subscription)
+      }
     }
 
     ws.onmessage = event => {

src/types/index.ts

@@ -418,6 +418,7 @@ export interface PubsubSubscription {
 }
 
 export interface PubsubMessageHandler {
+  onOpen?: (subscription: PubsubSubscription) => void
   onMessage: (message: Bytes, subscription: PubsubSubscription) => void
   onError: (error: BeeError, subscription: PubsubSubscription) => void
   onClose: (subscription: PubsubSubscription) => void