Add K8s Consul Auth Support

Came up in the issue about adding documentation for the k8s support we do have (which is Vault only). https://github.com/hashicorp/consul-template/issues/1638

Basically add support for K8s Consul Auth just like is supported for Vault... and be sure to document.

Start by reading the related issue #1638 and locating the existing Kubernetes Vault authentication support in consul-template. Determine the corresponding Consul authentication flow and document the new support; done means both the implementation and its usage documentation are covered.

Written by the indexing model from the issue text.