A handler is a function that performs the actual transformation of a command
and request into a result. A handler typically sends HTTP requests. Handlers
can be composed with middlewares to augment their behavior. A handler is a
function that accepts an ``Aws\CommandInterface`` and a
``Psr\Http\Message\RequestInterface`` and returns a promise that is fulfilled
with an ``Aws\ResultInterface`` or rejected with an
``Aws\Exception\AwsException`` reason.