Option to set static UDP port for magicsock #5175
Description
When the agent starts the magicsock port seems to be selected randomly which require a wider port range in a firewall configuration (maybe the problem is related to the public ip being present?).
2022-11-27 21:24:34.859 [DEBUG] (tailnet.wgengine) <./tailnet/conn.go:712> Logger.func1 magicsock: endpoints changed: 123.123.123.123:48483 (local)
2022-11-27 21:24:34.872 [DEBUG] (tailnet) <./tailnet/conn.go:209> NewConn.func3 wireguard status {"status": "\u0026{AsOf:2022-11-27 21:24:34.872024622 +0000 UTC m=+1.317492057 Peers:[] LocalAddrs:[{Addr:123.123.123.123:48483 Type:local}] DERPs:1}", "err": null}
2022-11-27 21:24:34.875 [DEBUG] (tailnet) <./tailnet/conn.go:209> NewConn.func3 wireguard status {"status": "\u0026{AsOf:2022-11-27 21:24:34.875214234 +0000 UTC m=+1.320681647 Peers:[] LocalAddrs:[{Addr:123.123.123.123:48483 Type:local}] DERPs:1}", "err": null}
2022-11-27 21:24:34.905 [DEBUG] (tailnet.wgengine) <./tailnet/conn.go:712> Logger.func1 netcheck: [v1] report: udp=true v6=false v6os=true mapvarydest= hair= portmap= v4a=123.123.123.123:48483 derp=999 derpdist=999v4:22ms
2022-11-27 21:24:34.906 [DEBUG] (tailnet.wgengine) <./tailnet/conn.go:712> Logger.func1 magicsock: endpoints changed: 123.123.123.123:48483 (stun)
2022-11-27 21:24:34.906 [DEBUG] (tailnet) <./tailnet/conn.go:209> NewConn.func3 wireguard status {"status": "\u0026{AsOf:2022-11-27 21:24:34.906234275 +0000 UTC m=+1.351701690 Peers:[] LocalAddrs:[{Addr:123.123.123.123:48483 Type:stun}] DERPs:1}", "err": null}
another run
ss -tulpna
Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port Process
udp UNCONN 0 0 0.0.0.0:54900 0.0.0.0:* users:(("coder",pid=869,fd=15))
...
Would be great to have some kind of config param / env var which can be used.