Create new blosc2_schunk_decompress_chunk_ctx()

[FrancescAlted]

The existing blosc2_schunk_decompress_chunk() function uses the internal context associated with the superchunk for decompressing the specified chunk. However, reusing the same context is intrinsically thread unsafe.

It would be nice to add another blosc2_schunk_decompress_chunk_ctx() function where the user can pass a different context per thread, so that it can be thread safe.