Refactor group to extend domains that can be grouped (#40607)

Co-authored-by: Paulus Schoutsen <paulus@home-assistant.io>

homeassistant/components/binary_sensor/group.py

@@ -0,0 +1,15 @@
+"""Describe group states."""
+
+
+from homeassistant.components.group import GroupIntegrationRegistry
+from homeassistant.const import STATE_OFF, STATE_ON
+from homeassistant.core import callback
+from homeassistant.helpers.typing import HomeAssistantType
+
+
+@callback
+def async_describe_on_off_states(
+    hass: HomeAssistantType, registry: GroupIntegrationRegistry
+) -> None:
+    """Describe group on off states."""
+    registry.on_off_states({STATE_ON}, STATE_OFF)