Move builtin signals to enum (#2309)

* Move builtin signals to enum

* Fix annotations

sanic/mixins/signals.py

@@ -1,4 +1,5 @@
-from typing import Any, Callable, Dict, Optional, Set
+from enum import Enum
+from typing import Any, Callable, Dict, Optional, Set, Union
 
 from sanic.models.futures import FutureSignal
 from sanic.models.handler_types import SignalHandler
@@ -19,7 +20,7 @@ class SignalMixin:
 
     def signal(
         self,
-        event: str,
+        event: Union[str, Enum],
         *,
         apply: bool = True,
         condition: Dict[str, Any] = None,
@@ -41,13 +42,11 @@ class SignalMixin:
             filtering, defaults to None
         :type condition: Dict[str, Any], optional
         """
+        event_value = str(event.value) if isinstance(event, Enum) else event
 
         def decorator(handler: SignalHandler):
-            nonlocal event
-            nonlocal apply
-
             future_signal = FutureSignal(
-                handler, event, HashableDict(condition or {})
+                handler, event_value, HashableDict(condition or {})
             )
             self._future_signals.add(future_signal)