`ListTopicSubscriptionsPagedResponse.iterateAllElements()` should return `Iterable<Subscription>`

[jabubake]

ListTopicSubscriptionsPagedResponse.iterateAllElements currently returns Iterable<String>.
It feels inconsistent with ListTopicsPagedResponse.iterateAllElements that returns Iterable<Topic>
and ListSubscriptionsPagedResponse.iterateAllElements returns Iterable<Subscription>.

[garrettjonesgoogle]

This could get tricksy because of templating. Assigning to @michaelbausor to take a look at viability.

[garrettjonesgoogle]

We now have a ListTopicSubscriptionsPagedResponse.iterateAllAsSubscriptionName method which returns Iterable<SubscriptionName>. I think this satisfies the issue.

https://github.com/GoogleCloudPlatform/google-cloud-java/blob/d476ef7904467233e83168b8d1f5a934a0aae711/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/spi/v1/PagedResponseWrappers.java#L150