Add support for PassengerApi v2

[juschmitt]

As the title suggests, we should add support for the v2 API. v2 is less stable than v1. It is intended to have breaking changes to allow for removal of pending deprecations.

Would be nice to have some kind of feature flag that we can toggle when setting up the API to switch to v2. Maybe even on a "per request" basis?

We might get into trouble with our api models. Do we want to have separate models for v1 and v2?

[StefMa]

We might get into trouble with our api models. Do we want to have separate models for v1 and v2?

Regarding that, we could incorporate this with
#85

In general, I would recommend not just merge v1 and v2 into a single code base.
It should be separated IMO.
Consumers should be able to switch v1 and v2 with an dependency declaration.