[Feature] Add callback mechanism to evaluation

[acere]

I'm integrating fmeval with experiments tracking solutions (MLflow for now), and the lack of callback mechanisms means that the tracing can only happen after an evaluation is completed.
Drawbacks:

• results can be recorded only once the evaluation has completed (similar to [Feature] Streaming results/progress and summary metrics for faster feedback #278 )
• if the evaluation fails during the execution, the already generated values are lost

The suggested solution is to implement a callback mechanism to be able to tracks results as they're generated, simplifying integration with experiment tracking solutions.

[xiaoyi-cheng]

Thanks! Just let our PM know and we will consider add this to our roadmap.