Accepting contact as JSONObject

In transactions, when user is adding a transaction, currently system accepts `contact` as `Char`. This should be enhanced to provide a support for accepting this as `JSONObject` rather than `Char`.

For more information refer to iamhssingh/JyessTrading-FE#3

Look at the transaction endpoint where contact is currently a Char field. The change likely involves modifying the serializer or model to accept a JSONObject. Check the linked frontend issue iamhssingh/JyessTrading-FE#3 for context on the expected data structure. Test the updated endpoint to ensure it correctly parses and stores the JSON contact data.

索引モデルが issue の本文から書いたものです。