feature request(opensearch): allow disable json decoding in _df_doc_generator()

[AvihaiSam]

Is your feature request related to a problem? Please describe. I want the value to be indexed as string. instead it is indexed as an object

Describe the solution you'd like skip_json_decode argument which can be set True/False/list[str]. if list - it'd skip all the columns that are listed.

Describe alternatives you've considered use df.to_json(orient="records") and send using index_documents() instead