**Describe the bug**

blosc2_schunk_open_udio looks like it could be used to read an schunk from any seekable API (e.g. ftp), but it calls stat on the provided uri, requiring that the uri is known to the underlying OS filesystem API, which severely constrains where data can be source from. Providing any file is sufficient to trick this API into working, even if the file isn't a blosc2 compressed file. 

**To Reproduce**
TODO

**Expected behavior**
stat is not called, or stat is added as an optional callback in blosc2_io_cb

**Logs**
N/A

**System information:**
 - OS: Ubuntu 22.04
 - Compiler: clang
 - Version: fa5e3f4c5c645e315be0f998679b8e73b7d42680

**Additional context**
Add any other context about the problem here.
blosc2_schunk_open_udio calls stat
This is a GitHub Issue
repo:c-blosc2
owner:Blosc
Title : blosc2_schunk_open_udio calls stat
Issue date:

--- start body ---
**Describe the bug**

blosc2_schunk_open_udio looks like it could be used to read an schunk from any seekable API (e.g. ftp), but it calls stat on the provided uri, requiring that the uri is known to the underlying OS filesystem API, which severely constrains where data can be source from. Providing any file is sufficient to trick this API into working, even if the file isn't a blosc2 compressed file. 

**To Reproduce**
TODO

**Expected behavior**
stat is not called, or stat is added as an optional callback in blosc2_io_cb

**Logs**
N/A

**System information:**
 - OS: Ubuntu 22.04
 - Compiler: clang
 - Version: fa5e3f4c5c645e315be0f998679b8e73b7d42680

**Additional context**
Add any other context about the problem here.
--- end body ---
