FR: add some kind of identifying headers for tagged devices

### What are you trying to do?

I'd like to know where requests originate from, preferably without looking up IP addresses to match them to tailscale devices.

As the docs currently state on https://tailscale.com/kb/1312/serve#identity-headers

> These headers are not filled for traffic originating from [tagged devices](https://tailscale.com/kb/1068/acl-tags).

I'd like some other headers to DO get filled with the relevant tags, or maybe even the device name.

I have a single service that is being served both with a cloudflare tunnel and with the tailscale operator in ingress mode. This allows it to be publicly accessible with cloudflare handling the auth, but for devices on the tailnet it is possible to connect to it directly. However, if the tailscale headers don't get set on the requests, it is hard to determine where the request originated from.

### How should we solve this?

Add headers to the ingress mode that identify tagged devices as well. hostname + tags would be perfect.

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

I'd have to deploy my service twice, once with CF and once with tailscale, and add additional code to handle the deployment target. I'd prefer not to.

### Anything else?

I friggin love tailscale, I even got some stickers on my laptop! Great job so far :).