Support idempotency token for Storage client

[coryan]

The GCS API supports (or soon will support) an idempotency token header. The header allows GCS to detect if a request is a duplicate, and returns the same value for duplicate requests.

To implement this we need to:

• Create the unique id in the RetryClient
• The decorators below RetryClient need to accept a new "context" field on each function
• The gRPC and REST-based implementations need to consume this field and send it to the service as a header.

It is time to split the RawClient stack in two, like we do for all the other libraries. The *Connection stack will be used for mocks and to implement the retry loop. The typical function will look like so:

  virtual StatusOr<BucketMetadata> GetBucketMetadata(
      GetBucketMetadataRequest const& request) = 0;

The *Stub will implement logging, tracing, and actually sending the data to the service, the typical function will look like so:

  virtual StatusOr<BucketMetadata> GetBucketMetadata(
      rest_internal::RestContext& context,
      Options const& options,
      GetBucketMetadataRequest const& request) = 0;

Note how the options are passed explicitly and not tunneled via CurrentOptions. Also note that the "context" object is REST-based. We can change both over time, as the classes in the *Stub hierarchy are not needed for mocking.

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Java implemented this in:

https://github.com/googleapis/java-storage/pull/2027

Internally, the design doc is go/gcs-client-idem-token

[coryan]

I also need to stop using the idempotency policy, as these RPCs are now always idempotent.

[coryan]

This is blocked on service-side work.

[coryan]

Still blocked on service-side work.