rasterize_bins() can now produce a labels element

[ArneDefauw]

PR related to scverse/spatialdata-io#211 and provides complimentary changes to spatialdata.rasterize_bins.

See unit test https://github.com/ArneDefauw/spatialdata/blob/5c3edee642bad1eaedbddf2097430315b964bb1a/tests/core/operations/test_rasterize_bins.py#L36

And more in particular https://github.com/ArneDefauw/spatialdata/blob/5c3edee642bad1eaedbddf2097430315b964bb1a/tests/core/operations/test_rasterize_bins.py#L80

Note that even if the table is annotated by a labels layer created by spatialdata.rasterize_bins, we want to be able to run the raterize_bins function, but now withreturn_region_as_labels=True, see https://github.com/ArneDefauw/spatialdata/blob/5c3edee642bad1eaedbddf2097430315b964bb1a/tests/core/operations/test_rasterize_bins.py#L111.
There is a use case for this, e.g. you have leiden cluster ID's, and you want a 'snappy' way to also visualize gene expression levels in napari-spatialdata, see example below:

[codecov]

Codecov Report

Attention: Patch coverage is 94.04762% with 5 lines in your changes missing coverage. Please review.

Project coverage is 91.79%. Comparing base (7bce868) to head (8cd2304).
Report is 56 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata/models/_utils.py	64.28%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #811      +/-   ##
==========================================
- Coverage   91.80%   91.79%   -0.01%     
==========================================
  Files          45       45              
  Lines        7064     7118      +54     
==========================================
+ Hits         6485     6534      +49     
- Misses        579      584       +5     

Files with missing lines	Coverage Δ
src/spatialdata/__init__.py	96.29% <100.00%> (ø)
src/spatialdata/_core/operations/rasterize_bins.py	93.07% <100.00%> (+3.18%)	⬆️
src/spatialdata/transformations/_utils.py	91.92% <100.00%> (ø)
src/spatialdata/models/_utils.py	88.33% <64.28%> (-2.09%)	⬇️

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[LucaMarconato]

I reviewed the PR and it's ready to merge! Thanks again @ArneDefauw.
The main change that I made is that I do not modify the instance_key column, rather I add a new one. I also added an helper function to quickly make the table annotate the labels (using this new column).

Finally, I added tests to cover the case in which instance_key is of string dtype.