lib/protocol: Further interface refactor (#9396)

This is a symmetric change to #9375 -- where that PR changed the
protocol->model interface, this changes the model->protocol one.

lib/model/model_test.go

@@ -3631,11 +3631,11 @@ func testConfigChangeTriggersClusterConfigs(t *testing.T, expectFirst, expectSec
 	cc1 := make(chan struct{}, 1)
 	cc2 := make(chan struct{}, 1)
 	fc1 := newFakeConnection(device1, m)
-	fc1.ClusterConfigCalls(func(_ protocol.ClusterConfig) {
+	fc1.ClusterConfigCalls(func(_ *protocol.ClusterConfig) {
 		cc1 <- struct{}{}
 	})
 	fc2 := newFakeConnection(device2, m)
-	fc2.ClusterConfigCalls(func(_ protocol.ClusterConfig) {
+	fc2.ClusterConfigCalls(func(_ *protocol.ClusterConfig) {
 		cc2 <- struct{}{}
 	})
 	m.AddConnection(fc1, protocol.Hello{})