breaking changes to pubsub client

[Michael Darakananda]

To support new Pubsub features, we're making a couple small breaking changes.

The first change: CreateSubscription now takes SubscriptionConfig struct.

Instead of writing

  c.CreateSubscription(ctx, subName, topic, 10*time.Second, nil)

you would write

  c.CreateSubscription(ctx, subName, pubsub.SubscriptionConfig{

    Topic: topic,

    AckDeadline: 10*time.Second,

  })

This is done to support new config fields.

The second change: Subscription's Config and ModifyPushConfig methods will return/accept the config struct by value. Currently, neither method allows nil pointers so there is little change in behavior.

Michael