Is there any HPA like controller that can scale up/down pods on demand?
In this scenario I would look at admission Webhooks, a comprehensive way to interact with K8s API server that can modify the basic behavior of some Kubernetes features by representing a Dynamic admission control model.
It means that you can implement special controllers that can intercept Kubernetes API requests, and modify or reject them based on custom logic.Thus, you can create either ValidatingWebhookConfiguration or MutatingWebhookConfiguration admission webhooks; the only difference validating webhooks can reject a request, but they cannot modify the object they are receiving in the admission HTTP request, while mutating webhooks can modify objects by creating a patch that will be sent back in the admission HTTP response. I would propose to get more relevant information in this tutorial.
According to the above said, it might be feasible to scale up or down relevant deployment resource based on the mutating rules in MutatingWebhookConfiguration within AdmissionReview API object as described in the official K8s documentation.