Switch to Kubernetes client-go library

[nabokihms]

Dex can earn some benefits from it:

• There a lot of ready features in client-go like canceling requests, retrying requests, static typing errors, API QPS settings, etc.
• Many people already familiar with client-go. Switching to client-go will make the contribution process easier for them.
• Kubernetes community uses client-go a lot, which means this library is well tested.