Feature Request: Fluent setters for generated GoStructs

[DanG100]

For example,

dev := &ygot.Device{}
dev.GetOrCreateInterface("eth1")
  .WithEnabled(true)
  .WithMacAddress("example")
  .GetOrCreateSubinterface(0).GetOrCreateIpv4()
    .WithMTU(100)

Add flag to generate this.