More stuff for the common jar

Okay, so to get readsets I currently do:
```
do {
SearchReadsetsResponse response = genomics.readsets.search(request).setFields('...').execute();

for (Readset readset: response.getReadsets()) {
}
/* page token handling */
} while (/* i have a page token */)
```

And this code would change that to:
```
Pager<SearchReadsetsRequest, SearchReadsetsResponse> readsetPager = Pager.searchReadsets(genomics, RetryPolicy.NEVER_RETRY);

for (SearchReadsetsResponse response : readsetPager.fetchAll(request)) {
  for (Readset readset : response.getReadsets()) {
  }
}
```

Questions:
* How can I call setFields (and other setX methods) in the pager?
* Can we provide a default retry policy of never_retry?
* Can `fetchAll` return just an `Iterator<Readset>` vs `Iterator<SearchReadsetsResponse>` ? (and maybe have a method called `fetchResponses` that does the behavior you have now) - cause I forsee a lot of double for loops in the future without that
(Note: that version of fetchAll will have to not crash on the rare case when response.getObjects() is null)