push_notification: Fix invalid data arg to firebase_messaging.Message.

Earlier, we were passing invalid `data` argument to
`firebase_messaging.Message` which would result in an error while
sending E2EE push notification for android devices.

The API requires all keys and values in the `data` dictionary
to be strings. One of the value was an integer.

This commit fixes the bug by converting the values to str if they
are not.

Signed-off-by: Prakhar Pratyush <prakhar@zulip.com>

zerver/lib/push_notifications.py

@@ -1447,6 +1447,9 @@ APNsPriority: TypeAlias = Literal[10, 5, 1]
 
 @dataclass
 class PushRequestBasePayload:
+    # FCM expects in this type must always be strings. Non-string
+    # fields must be converted into strings in the FCM code path
+    # within send_e2ee_push_notifications.
     push_account_id: int
     encrypted_data: str