Added ability to specify which update bot need to receive and process while using polling mode (#617)

* provide allowed_updates in polling mode

aiogram/utils/handlers_in_use.py

@@ -0,0 +1,28 @@
+from itertools import chain
+from typing import List, cast
+
+from aiogram.dispatcher.dispatcher import Dispatcher
+from aiogram.dispatcher.router import Router
+
+INTERNAL_HANDLERS = [
+    "update",
+    "error",
+]
+
+
+def get_handlers_in_use(
+    dispatcher: Dispatcher, handlers_to_skip: List[str] = INTERNAL_HANDLERS
+) -> List[str]:
+    handlers_in_use: List[str] = []
+
+    for router in [dispatcher.sub_routers, dispatcher]:
+        if isinstance(router, list):
+            if router:
+                handlers_in_use.extend(chain(*list(map(get_handlers_in_use, router))))
+        else:
+            router = cast(Router, router)
+            for update_name, observer in router.observers.items():
+                if observer.handlers and update_name not in [*handlers_to_skip, *handlers_in_use]:
+                    handlers_in_use.append(update_name)
+
+    return handlers_in_use