Send event when add-on changes state (#2608)

* Send event when add-on changes state

* fix test

supervisor/homeassistant/const.py

@@ -0,0 +1,26 @@
+"""Constants for homeassistant."""
+from enum import Enum
+
+from ..const import CoreState
+
+MIN_VERSION = {"supervisor/event": "2021.2.4"}
+
+CLOSING_STATES = [
+    CoreState.SHUTDOWN,
+    CoreState.STOPPING,
+    CoreState.CLOSE,
+]
+
+
+class WSType(str, Enum):
+    """Websocket types."""
+
+    AUTH = "auth"
+    SUPERVISOR_EVENT = "supervisor/event"
+
+
+class WSEvent(str, Enum):
+    """Websocket events."""
+
+    ADDON = "addon"
+    SUPERVISOR_UPDATE = "supervisor_update"