Reduce GCS QPS on ArrayRecordReader open via ReadLast.

[copybara-service]

Reduce GCS QPS on ArrayRecordReader open via ReadLast.

Previously, opening a gs:// ArrayRecord shard constructed a new
google::cloud::storage::Client instance and issued 3 separate GCS HTTP GET
requests:

1. An unbounded GET at offset 0 during GcsReader construction solely to obtain
   the object size (and then aborted mid-stream).
2. A GET via reader->NewReader(size - 64KB) to read the Riegeli postscript.
3. A GET via reader->NewReader(footer_offset) to read the ArrayRecord footer
   (and in OffloadedChunkOffset::operator[] when index_storage_option is
   offloaded).

This change:

1. Shares a process-wide google::cloud::storage::Client singleton across
   ArrayRecordReader instances in array_record_module.cc.
2. Opens riegeli::GcsReader with google::cloud::storage::ReadLast(1 MiB) so
   the initial HTTP GET retrieves the total object size (Content-Range) and
   prefetches the last 1 MiB of the shard.
3. In ArrayRecordReaderBase::Initialize(), pulls the prefetched tail into
   reader's buffer (resetting the cursor to limit() so available() == 0 on
   close) and reads the postscript and footer (including offloaded chunk index
   lookups in OffloadedChunkOffset::operator[]) directly from reader's
   buffered tail via ReadChunk when footer_offset >= reader->start_pos().