Adds pubsub.ClientConfig to the configuration (#35)

The pubsub client has more configuration options that what was being
exposed by the watermill configuration structs.

This adds the missing pubsub.ClientConfig struct to the watermill
configuration structs and changes the client initialization to
NewClientWithConfig instead of NewClient.

pkg/googlecloud/publisher.go

@@ -56,6 +56,7 @@ type PublisherConfig struct {
 	// Settings for cloud.google.com/go/pubsub client library.
 	PublishSettings *pubsub.PublishSettings
 	ClientOptions   []option.ClientOption
+	ClientConfig    *pubsub.ClientConfig
 
 	Marshaler Marshaler
 }
@@ -113,7 +114,7 @@ func connect(ctx context.Context, config PublisherConfig) (<-chan *pubsub.Client
 		defer close(errc)
 
 		// blocking
-		c, err := pubsub.NewClient(context.Background(), config.ProjectID, config.ClientOptions...)
+		c, err := pubsub.NewClientWithConfig(context.Background(), config.ProjectID, config.ClientConfig, config.ClientOptions...)
 		if err != nil {
 			errc <- err
 			return

pkg/googlecloud/subscriber.go

@@ -85,6 +85,7 @@ type SubscriberConfig struct {
 	ReceiveSettings    pubsub.ReceiveSettings
 	SubscriptionConfig pubsub.SubscriptionConfig
 	ClientOptions      []option.ClientOption
+	ClientConfig       *pubsub.ClientConfig
 
 	// Unmarshaler transforms the client library format into watermill/message.Message.
 	// Use a custom unmarshaler if needed, otherwise the default Unmarshaler should cover most use cases.
@@ -419,7 +420,7 @@ func (s *Subscriber) subscription(ctx context.Context, subscriptionName, topicNa
 }
 
 func (s *Subscriber) newClient(ctx context.Context) (*pubsub.Client, error) {
-	client, err := pubsub.NewClient(ctx, s.config.ProjectID, s.config.ClientOptions...)
+	client, err := pubsub.NewClientWithConfig(ctx, s.config.ProjectID, s.config.ClientConfig, s.config.ClientOptions...)
 	if err != nil {
 		return nil, err
 	}