pubub: add support for min-duration-per-ack-extension

[turneand]

Feature description

Currently SubscriberConfigurationProperties supports specifying the "max-duration-per-ack-extension", what I would like to see is support for the "min-duration-per-ack-extension".

Currently we get a lot of "INVALID_ARGUMENT: Some acknowledgement ids in the request were invalid", and although there's an underlying issue for some of them (see https://issuetracker.google.com/330381101) resulting in duplicate messages, it has been identified that setting the min-duration-per-ack-extension to a higher value than the default can result in significant reductions in issues (as the error is coming from the modack process).