Add Scope::with_kernel_label and with_xla_cluster

[adamcrume]

This is to mirror support in https://www.tensorflow.org/api_docs/cc/class/tensorflow/scope#withkernellabel. The implementation will be similar to Scope::with_device, but setting an attribute rather than a dedicated field.

Unfortunately, we can't support with_assigned_device, yet, because the C API doesn't expose that setter.

[adamcrume]

Scope::with_kernel_label was added in #294.

[siyavash]

Scope::with_xla_cluster was added in #295.

[siyavash]

Hello @adamcrume
Are we waiting for any other implementation to close this issue?