`is_kernel` flag in VK

[federicobarbacovi]

The is_kernel flag is currently part of DataBusPropagationData. Investigate whether it should be part of the circuit instance instead.

[kashbrti]

DataBusPropagationData is in the public inputs of the current instance. is_kernel indicates whether the call_data/return_data were coming from a Kernel or an app circuit, and I guess public inputs are the way this is getting passed to the current instance.

[ledwards2225]

DataBusPropagationData is currently part of the VK. Once we remove the public component keys from DataBusPropagationData, it will only contain is_kernel. If we need is_kernel, it should just become part of the VK directly. I suspect we actually dont need it at all