feat(trashbin): Add backend dav property

[Pytal]

Summary

Allow requesting the backend for trash items

[
  {
    "fileid": "195",
    "displayname": "Folder",
    "trashbin-deletion-time": "1731537993",
    "trashbin-backend": "OCA\\Files_Trashbin\\Trash\\LegacyTrashBackend"
  },
  {
    "fileid": "148",
    "displayname": "Group folder",
    "trashbin-deletion-time": "1731538002",
    "trashbin-backend": "OCA\\GroupFolders\\Trash\\TrashBackend"
  }
]

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)