Support push & discovery of PODs

[n0rad]

PODs define a group of ACIs working together as a unit, and so we are building with dgr a list of ACI and a POD manifest defining this unit.

For a single ACI there is a mechanism to push (#418) and to discover versions but there is nothing around doing it with PODs.

To get around this at BlaBlaCar, we are pushing pod's ACI and pod manifest to our repository, with a custom process, and using ggn to do POD discovery and prepare the command line to run.

So we think POD should also be pushable and discoverable.

This issues makes cnt not compliant with the spec, since there is nothing around PODs for this.