pubsub: allow Ack/Nack outside Receive function

**Is your feature request related to a problem? Please describe.**
I noticed that the docs were changed (in #5718) to say that `Ack()`/`Nack()` must be called inside the `Receive()` callback. Unfortunately, I have several applications built around the pubsub package that predate this and currently violate it.

**Describe the solution you'd like**
I was wondering if it would be possible to support the pattern of calling `Ack()`/`Nack()` outside of the `Receive()` callback. The reason is we want to handle messages in batches. Not all of our pubsub based applications benefit from working this way, but several do.

**Describe alternatives you've considered**
I haven't tried to reimplement these applications yet to meet the changed requirement, but that's what I'll be trying, if the pattern can't be supported!