add support for layer selection in get_values() method

[Sonja-Stockhaus]

We need the method in spatialdata-plot and I'm currently adding the possibility to color shapes/points/labels by values stored in a different layer of a table (scverse/spatialdata-plot#326). For that, we would need this functionality here as well.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.78%. Comparing base (2865dfd) to head (60193de).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata/_core/query/relational_query.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #818      +/-   ##
==========================================
- Coverage   91.79%   91.78%   -0.01%     
==========================================
  Files          45       45              
  Lines        7120     7124       +4     
==========================================
+ Hits         6536     6539       +3     
- Misses        584      585       +1     

Files with missing lines	Coverage Δ
src/spatialdata/_core/spatialdata.py	90.78% <ø> (ø)
src/spatialdata/_core/query/relational_query.py	90.51% <80.00%> (-0.15%)	⬇️

[LucaMarconato]

Looks great thanks! I added a small test, now auto-merging.