add missing fields to json output for image ls command

[albestro]

With uenv image ls --json some useful uenv information are missing. In particular, this PR adds version and tag.

Before

{
  "date": "2025-12-16 10:06:44",
  "digest": "d159129469c2d7e5bbd0944d1c174f56064c97315bd1228d301fc573eba290e6",
  "id": "d159129469c2d7e5",
  "name": "ascent",
  "size": 5763575808,
  "system": "daint",
  "uarch": "gh200"
}

After

{
  "date": "2025-12-16 10:06:44",
  "digest": "d159129469c2d7e5bbd0944d1c174f56064c97315bd1228d301fc573eba290e6",
  "id": "d159129469c2d7e5",
  "name": "ascent",
  "size": 5763575808,
  "system": "daint",
  "tag": "v1",
  "uarch": "gh200",
  "version": "0.9.5"
}

This might be useful not only for #130 (even if I know that @bcumming is working on an alternative way to collect information for the bash completion), but also for the final user.

[bcumming]

This feature has already been added to main in #131

https://github.com/eth-cscs/uenv/blob/main/src/uenv/print.cpp#L123

As far as I can tell, this PR should show a conflict, but it doesn't.