spanner

This is motivated by #5703. Applications may need to persist the state of pagination iterators, for example, because they are implementing a web service that returns pages of results.

I suggested this API:

struct Page {
  std::vector<std::string> bucket_names;
  std::string reader_state;
};

Page BucketsPage(gcs::Client client, std::string reader_state) {
  auto reader = client.

What

Package document comment is missing.

It is useful to understand how to use the packages.