Traits
The MockStubTrait is used by generated mock stub classes which extent \Grpc\BaseStub
(https://github.com/grpc/grpc/blob/master/src/php/lib/Grpc/BaseStub.php)
It provides functionality to add responses, get received calls, and overrides the _simpleRequest
method so that the elements of $responses are returned instead of making a call to the API.