Add DataConnector Interface

[vijusudhi]

DataConnectors should have a common interface to interact with AutoLLM platform.

TODOs:

• Define the interface.
• Define the placeholders each task demands.
  For example: QA task would demand {question}, {context} placeholders in the input template. Classification task would demand {possible labels}, {input} as placeholders.
• Implement DataConnectors for selected tasks and datasets. (use the exiting builder for PICO and PubMedQA for this)
• DataConnectors should finally export datasets to LLM4KMU HF org or store them as DatasetDict locally.