[experimental] z support get_dims for shapes

[LucaMarconato]

Experimental support of z dimension for shapes.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.19%. Comparing base (30d29bb) to head (8786a46).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata/models/_utils.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #997      +/-   ##
==========================================
- Coverage   92.20%   92.19%   -0.02%     
==========================================
  Files          49       49              
  Lines        7560     7562       +2     
==========================================
+ Hits         6971     6972       +1     
- Misses        589      590       +1     

Files with missing lines	Coverage Δ
src/spatialdata/models/_utils.py	89.20% <50.00%> (-0.46%)	⬇️

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