Add mod_time to /debug/ls

What does this MR do and why?

This MR adds mod_time to the /debug/ls response. This might be useful to understand when the orphaned data appeared

How to set up and validate locally

$ curl -s "http://localhost:6061/indexer/debug/ls" | jq .
[
  {
    "path": "/Users/dgruzd/projects/gdk-ee/zoekt-data/development/index/7_v16.00000.zoekt",
    "size": 582790,
    "mod_time": "2024-07-04T18:11:06.55945984+02:00"
  },
  {
    "path": "/Users/dgruzd/projects/gdk-ee/zoekt-data/development/index/node.uuid",
    "size": 36,
    "mod_time": "2024-04-25T12:51:29.132193404+02:00"
  }
]