Changes to Add a watchlist entry

the request property chain became nullable

the request property contract_address became nullable

the request property id became nullable

the request property slug became nullable

the request property token_id became nullable

added the media type */* for the response with the status 400

added the media type */* for the response with the status 404

added the optional property code to the response with the 403 status

added the optional property code to the response with the 409 status