Added the option to use a transport configured in another way

Currently, the configuration only allows to give the name of a transport class, which will be instantiated.
However, https://github.com/juriansluiman/SlmMail for example, has a factory that returns a transport. That factory uses configuration of its own to create the transport.
So the name of the SlmMail factory is not a class name to instantiate, but rather (returns) an already existing transport.

This PR allows passing an instantiated transport object in the config.