FR: Support for Kubernetes Auth Method

[patshash]

Currently Vault Benchmark supports tokens for authentication into the cluster. However it would be great if Vault Benchmark could use Kuberenets Auth method. It is difficult to run Vault Benchmark in any sort of automated fashion using a token as you've got to cycle the token on some schedule. If support was added for the Kubernetes authentication method then it would be simple to run the benchmark tool in an automated pipeline for testing and quality control purposes. Thanks.