support for deploy.update_config

[savv]

Is your feature request related to a problem? Please describe.
update_config configures how the service should be updated, and it particularly useful for rolling updates. podman-compose does not have support for it (and neither does docker-compose for that matter, except perhaps swarm. Adding this feature would be a clear reason to switch from docker-compose!)

Describe the solution you'd like
Implement this part of the spec.

Describe alternatives you've considered
This looks like an interesting alternative project https://github.com/evolutics/kerek Maybe there's something to be learned?