Support idempotency token for Storage client

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:

```cc
virtual StatusOr 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:

```cc
virtual StatusOr 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.

----

Java implemented this in:

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

Internally, the design doc is [go/gcs-client-idem-token](http://goto.google.com/gcs-client-idem-token)