Track device IDs for pushers (#13831)

Second half of the MSC3881 implementation

synapse/push/__init__.py

@@ -117,6 +117,7 @@ class PusherConfig:
     last_success: Optional[int]
     failing_since: Optional[int]
     enabled: bool
+    device_id: Optional[str]
 
     def as_dict(self) -> Dict[str, Any]:
         """Information that can be retrieved about a pusher after creation."""
@@ -130,6 +131,7 @@ class PusherConfig:
             "profile_tag": self.profile_tag,
             "pushkey": self.pushkey,
             "enabled": self.enabled,
+            "device_id": self.device_id,
         }