Add trusted proxies support

[james-atkins]

The range of proxy servers from which requests should be trusted (trusted_proxies) can be extended with plugins to maintain a dynamic list of IP ranges. It would be nice if we could use Tailscale tags for this.

eg something like this

trusted_proxies tailscale tag:servers

which would trust requests from machines with the servers tag.

[willnorris]

Ooh, cool! I hadn't ever really looked at that, but that sounds like a great idea. If someone wants to take a stab at doing that, go for it, otherwise I'll take a look at some point.

[bjeanes]

This would be very convenient!