blosc2_schunk_open_udio calls stat

[froody]

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: fa5e3f4

Additional context
Add any other context about the problem here.