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 ❤️

从 blosc2.save 周围的 Python API 和现有的基于 offset 的加载支持开始，然后比较 C 函数 blosc2_schunk_append_file 是如何暴露的。该 issue 没有明确是添加直接 API 还是 save mode；完成的标准是提供一条受支持的 append 路径，并覆盖将多个数组写入文件的情况。

由索引模型根据 Issue 内容生成。