Add container system df command for disk usage reporting

[realrajaryan]

• Closes [Request]: Storage - Add container system df command to show storage usage for images, containers, and volumes #884.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

This PR implements the container system df command to display disk usage statistics for images, containers, and volumes, along with their total count, active count, size, and reclaimable space for each resource type.

Active resources are determined by container mount references and running state, while reclaimable space is calculated from inactive or stopped resources.

Example output:

~/container ❯ container system df
TYPE           TOTAL  ACTIVE  SIZE      RECLAIMABLE
Images         4      3       4.42 GB   516.5 MB (11%)
Containers     4      2       2.69 GB   1.51 GB (56%)
Local Volumes  3      2       208.5 MB  66.2 MB (32%)

I'll have some follow-on PRs that will add -v/--verbose flag for detailed per-resource information, --filter flag for filtering output by resource type, and a --debug flag for debug statistics like block usage, clone counts etc.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

[dcantah]

Neat!