FR: Support manually specify endpoints

### What are you trying to do?

I have a server which has two different in/out public IP as a tailscale node. I'm trying to make other nodes directly connect to this server. The server has an in IP(such as 1.1.1.1) which accept in&out connection. Also the server has an out IP(such as 1.1.1.2) which accept out connection but reject in connection. Now i found the Endpoints ip showen in tailscale Machines page is out IP(1.1.1.2:41641). So other nodes will try to connect the out IP. Sadly, this out IP(1.1.1.2) will reject all active in connection. So the direct connection cannot be set up.

### How should we solve this?

Add an option to manually specify endpoints you'd like your device to advertise.

And support manually delete endpoints which are known to be impossible to connetc.

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

Right now other nodes cannot set up a p2p connetcion to this server, a traffic will go through DERP.
I manually add the in IP to the tailscale0 interface like #4958. And it works. But after i reboot the server, i have to add the in IP again. Sometimes the IP just disappears.

### Anything else?

_No response_