Support for router 'extra' attributes in topology definition

It would be nice to have possibility to add 'extra' attributes (memory, cpu) for routers in a topology definition. For example:

routers:
  - name: router
    base_box:
      image: munikypo/debian-10
    flavor: csirtmu.tiny1x2
    extra:
      memory: 512

Currently throwing error:

Error parsing <class 'kypo.topology_definition.models.Router'>, found key `extra` but expected any of dict_keys(['name', 'base_box', 'flavor'])