add ability to extract a JSON stringified key using response_transformer plugin

[tarciosaraiva]

Summary

Our case is around the integration of Kong + AWS Lambda plugin. When an error occurs in the Lambda end, the full response sent back by the Lambda is displayed on the response body, which includes the stack trace and the type of error.

Here's an example:

{
    "errorMessage": "{\"errorCode\":8,\"errorMessage\":\"actual error message\"}",
    "errorType": "java.lang.Exception",
    "stackTrace": [
        "sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)",
        "sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)",
        "sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)",
        "java.lang.reflect.Constructor.newInstance(Constructor.java:423)",
        "clojure.lang.Reflector.invokeConstructor(Reflector.java:180)",
        "shared.lambda$format_error.invokeStatic(lambda.clj:14)",
        "shared.lambda$format_error.invoke(lambda.clj:8)",
        "shared.lambda$handle.invokeStatic(lambda.clj:28)",
        "shared.lambda$handle.invoke(lambda.clj:17)",
        "clazzz.lambda$G__10962handleRequest.invokeStatic(lambda.clj:7)",
        "clazzz.lambda$G__10962handleRequest.invoke(lambda.clj:6)",
        "clazzz.lambda.handler.handleRequest(Unknown Source)"
    ]
}

The payload above contains way too much info and what we really want to send back is the contents of the errorMessage key. Since this is JSON content, we thought we could use the response-transfomer plugin to extract that information and send it back but the plugin does not support extraction.

Would be great if it did.