rust: add GCS listing and reading

[wchargin]

Summary:
This patch implements the extent of the Google Cloud Storage protocol
that TensorBoard needs: list objects in a bucket with a given prefix,
and read partial contents of an object. It turns out to be really easy.

For comparison, TensorFlow also rolls its own GCS client. Theirs
is more complex because it needs to handle writable files and support
general-purpose caching patterns. By contrast, we have a simple one-pass
read pattern and already assume that files are append-only, so we avoid
both the complexity and pathological interactions like #1225.

For now, this only serves public buckets and objects. Authentication is
also easy (and doesn’t require crypto or anything complicated), but, for
ease of review, we defer it to a future patch.

Test Plan:
Included a simple client that supports gsutil ls and gsutil cat. Run
with RUST_LOG=debug cargo run --release --bin gsutil and more args:

• ls tensorboard-bench-logs to list all 33K objects in the bucket,
  across 34 pages of list operations (3.3s on my machine);
• ls tensorboard-bench-logs --prefix mnist/ to list just a single
  logdir, which should be much faster (0.1 seconds on my machine,
  which includes setting up the keep-alive connection);
• ls tensorboard-bench-logs --prefix nopenope/ to check the case
  where there are no matching results;
• cat tensorboard-bench-logs mnist/README --to=11 to print the first
  12 bytes (Range: bytes=0-11 inclusive) of an object;
• cat tensorboard-bench-logs mnist/README --from=9999 to print
  nothing, since the object is shorter than 9999 bytes.

wchargin-branch: rust-gcs-client