Move google.cloud.iterator to google.api.core.page_iterator (#3770)
Move google.cloud.iterator to google.api.core.page_iterator (#3770)

* Move google.cloud.iterator to google.api.core.page_iterator
* Re-write tests to pytest style.
* Make GAXIterator private- it will soon be removed.
* Pass api_request into HTTPIterator to avoid accessing private members
* BigQuery: use google.api.core.page_iterator
* DNS: use google.api.core.page_iterator
* Logging: use google.api.core.page_iterator
* PubSub: use google.api.core.page_iterator
* Resource manager: use google.api.core.page_iterator
* Runtimeconfig: use google.api.core.page_iterator
* logging: use google.api.core._GAXIterator
* Storage: use google.api.core.page_iterator
* Pubsub: use google.api.core._GAXIterator
* Trace: use google.api.core._GAXIterator
* Spanner: use google.api.core._GAXIterator

googleapis/python-runtimeconfig,googleapis/python-runtimeconfig