Implement the interface for the external domain-specific models

Now the integration with external model is copmlicated because a lot of manual operations should be done.

Add new operation ("custom_model") to FEDOT. It will save the domain-specific model output (e.g. NEMO forecast) at fit stage and return in at predict stage. The real integration with NEMO is not required now, but the train/test separation should be supported;
Build the example with hybrid chain (ML models + NEMO);