Add Task (minimal implementation)

[alvarobartt]

Description

This PR adds the Task and TextGeneration classes, defining the custom behaviour for the Task and also TextGeneration which is the default task that basically runs the generation using an LLM and putting the results into a Python dictionary with a single key.

Note this is a minimal implementation of the Task and may be rewritten once the components of the Pipeline are ready.

What's missing?

• LLM refactor to include the load method and to update the LLM.generate method
• Decide whether {input,output}_mapping should be moved to Step.connect or as a Pipeline runtime arg