Provide a mechanism for authenticating to k8s using environment variables

[pablochacin]

xk6-disruptor requires interaction with the Kubernetes cluster in order to inject the faults in the targets. In order to do so, it needs to authenticate to the Kubernetes cluster.

Presently, this authentication requires either a kubeconfig file, or a service account mounted as a secret if running in-cluster (for example, using the k6-operator).

There are other usage scenarios, for example, when running in a CI/CD worker, on which neither of those methods is convenient.

Therefore, it would be convenient to be able to configure the credentials from environment variables.

For example, authenticate given the URL to the API server and a bearer token.