Load Xenium mask labels using Dask

[marcovarrone]

Towards #228

Replace the code to load the Xenium segmentation masks to use Dask to greatly reduce the memory consumption of the Xenium reader.

I abandoned the idea of saving each element to free up memory because the Xenium reader is already mostly Dask-compatible, so it will not bring any advantage.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 37.81%. Comparing base (7cccdd1) to head (0c80218).
⚠️ Report is 27 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_io/readers/xenium.py	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (7cccdd1) and HEAD (0c80218). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (7cccdd1)	HEAD (0c80218)
	3	2

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #337       +/-   ##
===========================================
- Coverage   49.06%   37.81%   -11.26%     
===========================================
  Files          26       26               
  Lines        2686     2721       +35     
===========================================
- Hits         1318     1029      -289     
- Misses       1368     1692      +324     

Files with missing lines	Coverage Δ
src/spatialdata_io/readers/xenium.py	19.11% <0.00%> (-53.53%)	⬇️

... and 5 files with indirect coverage changes

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