FR: Support for Kubernetes Auth Method

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.

Start by locating the existing token-based authentication flow in the vault-benchmark repository and reviewing how authentication is configured for automated runs. The work is done when the benchmark can authenticate with Vault's Kubernetes Auth method, allowing pipeline use without manually rotating a token.

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