Create dataset loader for EHRSQL

## Adding a Dataset
Name: EHRSQL
Description: Large-scale text-to-SQL dataset for question answering (QA) on electronic health records (MIMIC-III and eICU). It covers a wide range of questions asked in the hospital and aims to challenge the trustworthiness of the existing text-to-SQL models by testing whether the model can refuse to answer questions whenever they are not answerable.
Task: Text-to-SQL
Paper: [paper](https://openreview.net/forum?id=B2W8Vy0rarw)
Data: [data](https://github.com/glee4810/EHRSQL)
License: CC BY 4.0