Add command to list available package groups from package_manifest.yaml

[ogajduse]

Problem

There is no CLI command in obal that allows users to see which package groups (e.g., packages, repoclosures, copr_projects) are defined in package_manifest.yaml. This makes it harder to know which group targets are available for use with obal commands.

Proposal

Implement a command, e.g., obal list-groups, that prints all group names defined in package_manifest.yaml. This command should:

• Parse package_manifest.yaml.
• Output a list of all groups in a clear, human-readable format.

Implementation notes

obsah's find_targets() already loads the inventory via Ansible's InventoryManager and collects both hosts (ansible_inventory.hosts.keys()) and groups (ansible_inventory.groups.keys()). A list-groups command could reuse this machinery, filtering out the implicit Ansible groups (all, ungrouped) to return only the user-defined groups from the inventory file.

This is analogous to #426 (list packages), but at the group level.