OIDC provider support + EBS volume -> PersistentVolume helper

Added

• Node security group ID as output of the kops-cluster module
• Helper module to provision EBS volumes as kubernetes persistent volumes
  (includes the ability to restore them from a snapshot)
• Possibility to configure an OIDC provider to the Kubernetes API (tested with
  Google and Github)

Changed

• We're not creating an S3 VPC endpoint automatically, however you can create
  it outside of the kops-cluster module using the exported vpc-id.