Parse JSON string in adapt()

[mitmul]

Thank you for this great framework for evaluating RAG systems.

When using RAGAS, I encountered the following error while trying to adapt metrics to a specific language because the content of the output key in the elements of the examples attribute in the Prompt class was in un-parsed JSON (=str). It would be great if this PR could fix this error. I would appreciate it if you could check. Here is the error:

/usr/lib/python3.10/site-packages/ragas/llms/prompt.py", line 154, in get_all_keys                                 
    for key, value in nested_json.items():                                                    
AttributeError: 'str' object has no attribute 'items'  

[shahules786]

got it, thanks @mitmul