tailcfg: add Hostinfo.ExitNodeID to report the selected exit node (#16625)

When a client selects a particular exit node, Control may use that as
a signal for deciding other routes.

This patch causes the client to report whenever the current exit node
changes, through tailcfg.Hostinfo.ExitNodeID. It relies on a properly
set ipn.Prefs.ExitNodeID, which should already be resolved by
`tailscale set`.

Updates tailscale/corp#30536

Signed-off-by: Simon Law <sfllaw@tailscale.com>

tailcfg/tailcfg_test.go

@@ -67,6 +67,7 @@ func TestHostinfoEqual(t *testing.T) {
 		"UserspaceRouter",
 		"AppConnector",
 		"ServicesHash",
+		"ExitNodeID",
 		"Location",
 		"TPM",
 		"StateEncrypted",
@@ -273,6 +274,21 @@ func TestHostinfoEqual(t *testing.T) {
 			&Hostinfo{IngressEnabled: true},
 			false,
 		},
+		{
+			&Hostinfo{ExitNodeID: "stable-exit"},
+			&Hostinfo{ExitNodeID: "stable-exit"},
+			true,
+		},
+		{
+			&Hostinfo{ExitNodeID: ""},
+			&Hostinfo{},
+			true,
+		},
+		{
+			&Hostinfo{ExitNodeID: ""},
+			&Hostinfo{ExitNodeID: "stable-exit"},
+			false,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.a.Equal(tt.b)