FR: Support running the Kubernetes operator API server proxy over HTTP

### What are you trying to do?

Currently, to utilize the API server proxy in auth mode, HTTPS has to be enabled on the tailnet. I've avoided doing so for my tailnet to keep machine names off the certificate transparency record, and I don't believe there are any security benefits from adding a TLS certificate here since the connection to the proxy would already be encrypted over Tailscale.

### How should we solve this?

Allow running the API server proxy over HTTP.

### What is the impact of not solving this?

I'm not using this feature at the moment because of this. I wouldn't mind enabling HTTPS on my tailnet as much if there was a way to restrict which machines could request certificates, but #5917 is still open.

### Anything else?

_No response_