Hi, Chintan
Actually, there was a similar request placed on this issue link[1], there is presently no hook within CloudSQL which would allow publishing to a PubSub topic at the moment. This feature request was submitted to our CloudSQL engineers and they are reviewing the possibility. Sadly, there is no ETA for the implementation at this time.
Please keep in mind that Feature requests are subject to the evaluation of the Team and how it fits into the Road map of the specific product.
I recommend that you bookmark the issue link for regular updates on the fix.