Store prompt in Answer and eval dataframe

[tstadel]

Is your feature request related to a problem? Please describe.
For evaluating and optimizing generative pipelines it's essential to know which prompt (incl. full context) was used for each result. E.g. was the relevant document part of the prompt?
Currently we only have this info for PromptNode in the debug output per pipeline run. However we are lacking it on the result (i.e. Answer) level. Additionally OpenAIAnswerGenerator doesn't even provide this information. Pipeline.eval does not store the PromptNode prompt, so any analysis of the EvaluationResult is lacking the prompt.

Describe the solution you'd like

• store prompt for PromptNode and OpenAIAnswerGenerator in Answer objects (e.g. meta or dedicated field)
• store prompt to EvaluationResult dataframes

Describe alternatives you've considered

Additional context
Add any other context or screenshots about the feature request here.