Response templating for ASP.NET Core API Gateway demo

[tmasternak]

Overview

In it's current form API Gateway demo makes sure that request handling logic (calling appenders that match url and assembling response from parts returned by appenders and subscribers is generic - does not depend on url and/or appenders).

Depending on the usecase it might be useful to support templating for api gateway responses. In such case the owner of api gateway contract would be able to have a bigger control over the structure of json document returned to the clients.

Connects to: #59