Storage clients shouldn't be required to provide a response decoder factory

I agree that the specific storage client should be responsible for how to decode the repsonse. So I'm in favour of moving it out of the base class and move it to the specific implementation.

I support that as well and as the developer of other external entity storage that are not using a response decoder factory, I had to include that without using it in my modules, which doesn't make sense. I'll update my modules without problem once the change is made to External entities module.