FR: Activate and choose an exit node from `tailscale web` UI

### What are you trying to do?

Use tailscale as geateway on router (pfsense,openwrt) and be able to easly switch exit point/location. Switching location not to chose specyfic country server in case this server its down

### How should we solve this?

easly switching location when using tailscale on router without accessing command line configuration

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

Nessesery to edit configuration on linux machine and define specyfic exit point-what is specyfic server is down and its critical to not disclose location

### Anything else?

This would be perfect solution when traveling/working with own router when its nessesary to hide current location and still access own private network all in one product