Expose `blosc2_schunk_append_file` to Python API [Feature request]

Hey and thanks in advance for the support!

I use blosc2 to write multiple small arrays into files, and I want to reduce the number of files.
I saw there is a C function that appends an SChunk to a file, `blosc2_schunk_append_file`, but couldn't find a way to call it from the Python API. Will it be possible to expose such function? or support it as a mode in `blosc2.save`?
We already have the option to load arrays from files with offset, so an option to append seems logical to me.

Thanks again ❤️

Start with the Python API around blosc2.save and the existing offset-based load support, then compare how the C function blosc2_schunk_append_file is exposed. The issue leaves open whether to add a direct API or a save mode; done means one supported append path with coverage for writing multiple arrays into a file.

Written by the indexing model from the issue text.