Implement __contains__ method for states group

aiogram/dispatcher/filters/state.py

@@ -91,6 +91,13 @@ class MetaStatesGroup(type):
     def childs(cls):
         return cls._childs
 
+    @property
+    def all_childs(cls):
+        result = cls.childs
+        for child in cls.childs:
+            result += child.childs
+        return result
+
     @property
     def all_states(cls):
         result = cls.states
@@ -106,6 +113,15 @@ class MetaStatesGroup(type):
     def states_names(cls) -> tuple:
         return tuple(state.state for state in cls.states)
 
+    def __contains__(cls, item):
+        if isinstance(item, str):
+            return item in cls.all_states_names
+        elif isinstance(item, State):
+            return item in cls.all_states
+        elif isinstance(item, StatesGroup):
+            return item in cls.all_childs
+        return False
+
     def __str__(self):
         return f"<StatesGroup '{self.__full_group_name__}'>"