We added a new feature to the GapicClientTrait adding the new method: pependMiddleware.

Adding a middleware involves passing a closure to the addMiddleware and prependMiddleware with the first parameter being a MiddlewareInterface like so:

$client->addMiddleware(function (MiddlewareInterface $handler) {...});

For prependMiddleware this is not always the case as the final call in the callStack is not a MiddlewareInterface but a closure.

This PR moves this closure into its own middleware so the $callStack variable is always of type MiddlewareInterface fixing the problem with the prependMiddleware class and adding more consistency to typing.