[M2] Begin implementation of the 'app2' module.

## Context
The `app2` module aims to improve on and replace the `app` module (eventually). For now, we should use `app2` to implement direct `dmsg` connections to the skywire hypervisor.

**Specifications:** https://github.com/watercompany/skywire-specs/tree/master/proposals/evanlinjin-app2