Now return an iterable over pages of the response resource field,
rather than the return type, for page stream-able calls where page
streaming is disabled.

Note that this removes the ability to get a standard gRPC/proto
response from any call that is capable of page streaming.

Now, if page streaming is set to per-page (rather than per-resource), it
is possible to specify a page token to the request. It is also possible
to retrieve the full response message, as well as the next page token,
from the per-page iterator.

Previously, assumed page_token was a string. Now it can have any type.