This makes it easier to implement dynamically sized types in go-marshal. You
really only need to implement MarshalBytes, UnmarshalBytes and SizeBytes to
implement the entire interface.
By using the `dynamic` tag, the autogenerator will generate the rest of the
methods for us.
This change also simplifies how KernelIPTGetEntries implements Marshallable
using the newly added utility.
PiperOrigin-RevId: 356397114
cfa4633c3d