Added ragas to compute answer metrics for evaluation. (#1039)

* Removed duplicate code in query execution.

* Added nltk download logic to support string metrics.

* Add ragas to imports.

* Added ragas to imports.

* Added string metrics for evaluation.

* Remove dthe unwanted break statement.

* Moves the import to top of file.

* Moved the scorer definitions to the init function.

* Refactoring.

* Removed unused imports.

* Moved async calls to the outer function.

* Refactor to add error handling and change function names.