Implement notification system for Config

I want cloud-init to depend on the Conf, and not vice-versa. So I must
somehow call the cloud-init utilities from the config. My solution:
callbacks

windows-agent/internal/config/config.go

@@ -32,6 +32,9 @@ type Config struct {
 
 	// Sync
 	mu *sync.Mutex
+
+	// notifiers are called after any configuration changes.
+	notifiers []func()
 }
 
 // New creates and initializes a new Config object.
@@ -44,6 +47,11 @@ func New(ctx context.Context, cachePath string) (m *Config) {
 	return m
 }
 
+// Notify appends a callback. It'll be called very time any configuration changes.
+func (c *Config) Notify(f func()) {
+	c.notifiers = append(c.notifiers, f)
+}
+
 // Subscription returns the ProToken and the method it was acquired with (if any).
 func (c *Config) Subscription(ctx context.Context) (token string, source Source, err error) {
 	c.mu.Lock()
@@ -130,6 +138,10 @@ func (c *Config) set(ctx context.Context, field *string, value string) error {
 	old := *field
 	*field = value
 
+	for _, f := range c.notifiers {
+		f()
+	}
+
 	if err := c.dump(); err != nil {
 		log.Errorf(ctx, "Could not update settings: %v", err)
 		*field = old
@@ -250,6 +262,10 @@ func (c *Config) collectRegistrySettingsTasks(ctx context.Context, data Registry
 		log.Warningf(ctx, "Could not obtain some updated registry settings: %v", errs)
 	}
 
+	for _, f := range c.notifiers {
+		f()
+	}
+
 	// Dump updated checksums
 	if err := c.dump(); err != nil {
 		return nil, fmt.Errorf("could not store updated registry settings: %v", err)