add suport for zstd compression of h5sc files

[sophiamaedler]

I ran a quick benchmark for writing h5ad files with lzf compression vs zstd compression:

Dataset	Write LZF (s)	Write ZSTD (s)	Read LZF (s)	Read ZSTD (s)	Read LZF (backed)	Read ZSTD (backed)	LZF .h5ad (GB)	ZSTD .h5ad (GB)
Dataset A	63.79	74.60	31.14	19.30	28.94	18.03	2.9	2.1
Dataset B	64.28	70.41	30.83	19.37	28.46	18.04	2.8	2.0
Dataset C	61.01	72.50	30.68	18.93	28.31	17.40	2.8	2.0
Mean ± SD	63.03 ± 1.7	72.50 ± 2.1	30.88 ± 0.2	19.20 ± 0.2	28.57 ± 0.3	17.82 ± 0.3	2.83 ± 0.05	2.03 ± 0.05

Metric	ZSTD / LZF ratio	Relative difference	Interpretation
Write time	1.15×	+15 % slower	Slight write-speed penalty
Read time	0.62×	−38 % faster	Much faster read
Read (backed)	0.62×	−38 % faster	Same gain in backed mode
File size	0.72×	−28 % smaller	~30 % storage saving

zstd compression has the downside that we are dependent on an additional plugin (hdf5plugin) for both reading and writing making the files slightly less broadly compatible. Nevertheless it would be interesting to add zstd support to scPortrait as this could be quite relevant to some users.