Insecure run modes should be either not possible or explicitly opted in to

Right now, network proxy server supports three auth modes (if I read https://github.com/kubernetes-sigs/apiserver-network-proxy/blob/2fdb1a46954b1e99af989b037e543751eae49fe0/cmd/server/main.go correctly).

1. TLS Client auth
2. Service account token auth
3. None

The third mode should be ideally removed. If it is absolutely needed, it should be explicitly opted into via an e.g. `--insecure-no-client-auth` flag.

Start with cmd/server/main.go, which defines the proxy server's authentication modes, and trace how the TLS client, service account token, and unauthenticated modes are selected. Determine whether the unauthenticated mode can be removed or must require an explicit opt-in; done means insecure operation is unavailable by default and the chosen behavior is enforced.

Written by the indexing model from the issue text.