Feat: Add Go template equivalent of Krew `whoami`

### Expected behavior
When working in multi-developer environments, it is useful to have an "identity seed" for templating resources. A good example of this is when project developers share an external secrets namespace. Being able to retrieve the user identity from the current kubeconfig is useful to select from a sub-namespace for development secret storage.

Creating this as a property in the Go template space would allow it to be used (and transformed in a chained template).