Allow generating admin kubeconfig (k3s.yaml) with server hostname instead of IP address

[brandond]

Add a CLI flag, env var, or something else that allows generating the admin kubeconfig with the server's hostname in the server URL, instead of the loopback address or bind address.

Originally posted in #11165 (comment):

@krisgeus
I can use sed to fix the kubeconfig bit I also need the servername as alias in the certificate (which the tls-san does correctly) I would expect the server to write out a valid config I can use without fiddling with it myself.

@brandond
Note that our docs have ALWAYS referenced changing the server address in the admin kubeconfig if you want to acccess the server from elsewhere: https://docs.k3s.io/cluster-access#accessing-the-cluster-from-outside-with-kubectl