Support example responses for GET requests

[jlengrand]

This is a followup from #77

At the moment, only POST requests with a request body are supported and will see their example responses extracted.

The reason is that the PostmanRequestItem creation getPostmanRequests method only contains id in the case where codegenOperation.bodyParam isn't null.

This needs to be improved (but can be done just a bit later, since currently our GET requests don't contain example responses.

[jlengrand]

thinking again, I wonder if we need to do this now, for the Adyen generator @gcatanese ?

I feel like this should be done in the generic generator first, and then adapted for Adyen needs but today we won't get examples for GET requests will we?