Add type hint when get a SpatialElement item

[quentinblampey]

Currently, when we get an item by name, e.g. geo_df = sdata.shapes["boundaries"], then the type of geo_df is not inferred and we lose type hints and auto-completion.

This PR aims to fix this, via the use of TypeVar, to precise the type of each element.

I also added AnnData to the return types of SpatialData.__getitem__

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.11%. Comparing base (eb5a202) to head (8f0f242).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #939   +/-   ##
=======================================
  Coverage   92.11%   92.11%           
=======================================
  Files          48       48           
  Lines        7429     7432    +3     
=======================================
+ Hits         6843     6846    +3     
  Misses        586      586           

Files with missing lines	Coverage Δ
src/spatialdata/_core/_elements.py	92.39% <100.00%> (+0.16%)	⬆️
src/spatialdata/_core/spatialdata.py	91.46% <100.00%> (ø)
src/spatialdata/testing.py	98.14% <100.00%> (+0.03%)	⬆️

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[LucaMarconato]

LGTM thanks!