Not possible to pass extra options to Guzzle #16
Description
There are a number of use cases where it is necessary to pass extra options to the HTTP Client that the SDK uses, e.g. configuring a proxy, or even allowing the developer to pass their own instance of Guzzle\Client to allow for custom middlewear to be used.
Due to the fact that Sanity\Client uses private properties it isn't possible to easily extend the class to override this default behaviour.
Could the ability to do the above be added?