[FEATURE] Make possible to get response as a string

[anton-tarasiuk]

Is your feature request related to a problem? Please describe.
I've faced a case when response is needed to be used in next request "as is" - without converting it to object. I believe it could be useful to make response string available by adding default mapping for String type: String: jsonEncode,.

Describe the solution you'd like
Current solution is:

1. Use response_override_value_map to override YourResponseModel with String
2. Add String: jsonEncode, to generatedMapping rules.