Add safe sciebo file updating

[D-VR]

This supersedes PR #117

• Sciebo shares are now traversed via WebDAV PROPFIND with an explicit request for oc:checksums, so each file gets a stable content hash (SHA1) in addition to getetag.
• For each Sciebo item, we store the SHA1 checksum on the node as its etag and persist it in the per‑course .syncmymoodle_cache alongside the usual Moodle nodes.
• During downloads, we use the cached Sciebo checksum to detect remote changes: if the SHA1 from the new PROPFIND matches the old cached one, we skip re‑downloading that file.
• When the remote checksum changed, we hash the local file and compare it to the old checksum to detect local modifications; if they differ, we treat this as a sync conflict and apply the same
  update_files_conflict strategies (rename, keep, overwrite) as for Moodle files.
• If there is no cache yet but a local file already exists, we compare its hash against the current Sciebo checksum from PROPFIND; if they match, we treat the file as up‑to‑date and skip both downloading and
  conflict handling.

This solves #110
This PR (should) also fix #74 by checking the response header when resuming partially downloaded files
And as far as I can tell, the current code should resolve #35