support on prompt event handler (#765)

Co-authored-by: Lt.Dr.Data <lt.dr.data@gmail.com>

server.py

 import os
 import sys
 import asyncio
+import traceback
+
 import nodes
 import folder_paths
 import execution
@@ -88,6 +90,8 @@ class PromptServer():
         self.last_node_id = None
         self.client_id = None
 
+        self.on_prompt_handlers = []
+
         @routes.get('/ws')
         async def websocket_handler(request):
             ws = web.WebSocketResponse()
@@ -438,6 +442,7 @@ class PromptServer():
             resp_code = 200
             out_string = ""
             json_data =  await request.json()
+            json_data = self.trigger_on_prompt(json_data)
 
             if "number" in json_data:
                 number = float(json_data['number'])
@@ -606,3 +611,15 @@ class PromptServer():
         if call_on_start is not None:
             call_on_start(address, port)
 
+    def add_on_prompt_handler(self, handler):
+        self.on_prompt_handlers.append(handler)
+
+    def trigger_on_prompt(self, json_data):
+        for handler in self.on_prompt_handlers:
+            try:
+                json_data = handler(json_data)
+            except Exception as e:
+                print(f"[ERROR] An error occurred during the on_prompt_handler processing")
+                traceback.print_exc()
+
+        return json_data