Provide access to HttpApiTransport._client

[tjoubert]

Expose a readonly getter for HttpClient on HttpApiTransport (e.g. InnerClient) to allow callers to use the static creation methods for convenience, and then set a few options on the client themselves. Example:

var transport = HttpApiTransport.UsingBasicAuth(/*...*/);
transport.InnerClient.Timeout = TimeSpan.FromMinutes(2);