Add formatting_fn arg in Engine and all the subclasses

[alvarobartt]

As internally discussed with @dvsrepo and @gabrielmbmb, we should add an arg in the Engine base class (former LLM class) to allow users to provide their own specific formats to send to the model, in order to detach that functionality from the Task (former PromptTemplate), which implies that users can use any available LLM available in Hugging Face even if there's not a default supported format.