gke-disk-image-builder: Configurable shielded instance config

Customer requirement: need to be able to configure the compute instance to run using shielded settings:

What I needed:

```go
+   ShieldedInstanceConfig: &compute.ShieldedInstanceConfig{
+           EnableSecureBoot:          true,
+           EnableVtpm:                true,
+           EnableIntegrityMonitoring: true,
+   },
```

- @nstogner
