Add API data source

[schuderer]

There should be data source(s) for querying other RESTful APIs.

How this should be solved, whether in MLLP core or as an extra package, and how generic this solution should be, is all still open to discussion. We cannot make it compatible with all and every possible API. So we may need to provide a base class/tool kit to make it easier to create specific API data sources..?

By having this, we can potentially use on-premise APIs (data core, ...), or public APIs (weather, twitter, ...)

Also, we can potentially increase modularity between small model APIs, being able to re-use stuff that is already in production (think of generic services like sentiment recognition, pdf/image ocr, ...).
Of course, this also creates interdependence and these re-used APIs need to be really REALLY stable.