Apply custom JS payload mapper for responses of Http endpoints in http-push connections

[thjaeckle]

Currently, the HTTP response of a managed http connection can be used as response to a message forwarded by Ditto to that foreign endpoint.

Payload mapping can even be used to alter the request payload.
The response payload, however, can not be altered currently.

In oder to be able to do that, the target of a Http push connection would have to define a response payload mapper.
In case of JS mapper, we need a new function, mapping from externalMessage to externalMessage.