Support sharding through config and raster_write_kwargs

[melonora]

This PR adds the following:

• passing kwargs for zarr.create_array directly as raster_write_kwargs for io functions like .write and .write_element. This also adds the ability to write sharded arrays. Support for anndata sharding is to be added in a follow up PR.
• proper docstrings for the new raster_write_kwargs argument.
• Extension of the current config to include raster_chunks and raster_shards. The config can now be stored in a default location or a custom location. Additionally, environment variables can be set to temporarily override the values.
• Adding zarrs as a dependency and enabled the codec by default to allow for faster io when writing shards. This is a discussion point of whether we should do this or provide more of an opt-in for advanced users.

Additional changes

• Minimal supported version of dask is 2026.3.0. The reason here is that only this provides the api in such a way that you don't risk zarr format 2 being written in a zarr v3 group and vice versa + it includes the setting that prevents collaps of partitions of dask dataframes after reading from parquet.

@LucaMarconato

[melonora]

Failing atm due to ome-zarr not yet being released. You can test locally with ome-zarr-py from main.

Also, need to add support for zarrs to improve speed of shard io

[codecov]

Codecov Report

❌ Patch coverage is 90.26549% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.95%. Comparing base (cf91ad5) to head (03aafc9).

Files with missing lines	Patch %	Lines
src/spatialdata/config.py	87.50%	9 Missing ⚠️
src/spatialdata/_core/_utils.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1106      +/-   ##
==========================================
+ Coverage   91.93%   91.95%   +0.02%     
==========================================
  Files          51       51              
  Lines        7772     7881     +109     
==========================================
+ Hits         7145     7247     +102     
- Misses        627      634       +7     

Files with missing lines	Coverage Δ
src/spatialdata/__init__.py	96.00% <100.00%> (+0.34%)	⬆️
src/spatialdata/_core/spatialdata.py	92.01% <100.00%> (+0.08%)	⬆️
src/spatialdata/_io/io_raster.py	94.59% <100.00%> (+2.50%)	⬆️
src/spatialdata/_core/_utils.py	97.01% <89.47%> (-2.99%)	⬇️
src/spatialdata/config.py	88.60% <87.50%> (-11.40%)	⬇️

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[Mr-Milk]

Should we also allow the control of sharding for anndata?

[melonora]

Yes, but not as part of this PR. I will adjust the config though to accommodate.