Support SW custom function definitions

It will be nice if users, through addons, can add custom function definitions to Serverless Workflow.
This will be handy to avoid repetitive function definition when the same API is used in several files and also to add common argument and return object manipulations for a distinguished api (for example, Ansible).
The idea is to provide, inside Kogito engine, a set of interfaces that users can implements in their addons to define these custom function definitions.