Reduce reader memory consumption

[marcovarrone]

It adds a new output_path parameter in a reader's function that allows to save every element of the spatialdata object as soon as it's created.
This frees up part of the memory during the function's execution instead of maintaining the whole spatialdata object in memory and then saving it with sdata.write.

I've done it only for Xenium but if I receive the ok I can implement it for the rest of the readers.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 37.25490% with 32 lines in your changes missing coverage. Please review.

Project coverage is 45.02%. Comparing base (cff4896) to head (43f0d04).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_io/readers/xenium.py	37.25%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #228      +/-   ##
==========================================
- Coverage   45.50%   45.02%   -0.48%     
==========================================
  Files          22       22              
  Lines        2149     2183      +34     
==========================================
+ Hits          978      983       +5     
- Misses       1171     1200      +29     

Files with missing lines	Coverage Δ
src/spatialdata_io/readers/xenium.py	66.18% <37.25%> (-5.61%)	⬇️