agent: Use GOOPS

GOOPS is Guile Object Oriented Programming System.

It's easier to extend and break less the scenarios using GOOPS.  e.g., a
scenario published on Gitlab should work two weeks later.  This is not the case
right now and the goal of using GOOPS is to offer a stable API for the agent,
while keeping it flexible.

Change-Id: If6a038b6d8c371f8e74849749770f1dec8559b91