net/sockstats: remove explicit dependency on wgengine/monitor

Followup to #7177 to avoid adding extra dependencies to the CLI. We
instead declare an interface for the link monitor.

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/interfaces/interfaces.go

@@ -353,6 +353,12 @@ func (s *State) String() string {
 	return sb.String()
 }
 
+// ChangeFunc is a callback function (usually registered with
+// wgengine/monitor's Mon) that's called when the network
+// changed. The changed parameter is whether the network changed
+// enough for State to have changed since the last callback.
+type ChangeFunc func(changed bool, state *State)
+
 // An InterfaceFilter indicates whether EqualFiltered should use i when deciding whether two States are equal.
 // ips are all the IPPrefixes associated with i.
 type InterfaceFilter func(i Interface, ips []netip.Prefix) bool