feat: Add pod_network setting for pod network inheritance

[cyberactive-paramount]

When using pods with external networks (such as macvlan for routable IPs), containers need to inherit the pod's network namespace rather than having their own network configuration. Previously, podman-compose always added --network=_default to container commands, which overrode podman's automatic network namespace inheritance from the pod.

This change adds a new x-podman.pod_network setting that:

1. Specifies the network to attach to the pod during creation
2. Skips adding --network to container commands when containers should inherit from the pod

The feature enables use cases like:

• Macvlan networks where the pod has a routable LAN IP
• External networks managed outside of compose
• Containers that need to share localhost within the pod

Usage in docker-compose.yml:

x-podman:
  pod_network: my_external_network
  pod_args: "--infra=true --share=ipc,net"

services:
  myservice:
    image: myimage
    # No 'networks' or 'network_mode' - inherits from pod

The container will only inherit from pod when:

• pod_network is configured
• Container is assigned to a pod
• Container has no explicit network_mode or networks configuration