Multiple 'Set-Cookie' header within Header struct (VirtualService)

**Describe the feature request**
Regarding `type Headers_HeaderOperations struct`
https://github.com/istio/api/blob/master/networking/v1alpha3/virtual_service.pb.go#L793-L804

As the key of `Add` attribute reflects the header name, setting multiple keys/headers like `Set-Cookie` is impossible.
Is there a way to set multiple `Set-Cookie` headers like separating cookies definitions with `,` (tried and failed). This would require that istio would unfold this to separate header.

**Describe alternatives you've considered**

As I have the need to set 2 cookies right now I am doing kind of a workaround and set one within the `Http` struct and one into the `Destination`. This works and envoy delivers two separate `Set-Cookie` headers.

**Affected product area (please put an X in all that apply)**

[x] Configuration Infrastructure
[ ] Docs
[ ] Installation
[x] Networking
[ ] Performance and Scalability
[ ] Policies and Telemetry
[ ] Security
[ ] Test and Release
[ ] User Experience

**Additional context**

But we have the use case which would require to set more than 2 Cookies within one route.