Pub/Sub Spring Cloud Stream binder should support ackMode customization

[elefeint]

Is your feature request related to a problem? Please describe.
Currently stream binder creates a PubSubInboundChannelAdapter in default AUTO acknowlegement mode. As discussed in #2074, this does not allow a message to expire naturally -- a message will be acked in normal execution or in exception scenario where an error handler is declared, and a message will be nacked if an unhandled error occurs.

Describe the solution you'd like
Provide ability to override the PubSubInboundChannelAdapter ackMode property.

Describe alternatives you've considered
Another option discussed in original issue was to avoid raising an exception in AUTO_ACK and MANUAL modes. However, this would reduce the error handling flexibility, as it would eat the exception and provide no ability to recover its cause.