Faster imports

[LucaMarconato]

Replaces #372
To be merged after #370

Significantly faster import time: 25%-30% faster for specific readers and now lazy-loading when doing import spatialdata-io.

Results from asv (benchmark script also included):

| Change   | Before [7eb2e61a] <main>   | After [e7de0877] <faster-imports>   |   Ratio | Benchmark (Parameter)                                                      |
|----------|----------------------------|-------------------------------------|---------|----------------------------------------------------------------------------|
| -        | 2.34±0.06s                 | 1.70±0.03s                          |    0.73 | benchmark_imports.ImportBenchmark.time_from_spatialdata_io_import_visium   |
| -        | 2.39±0.07s                 | 1.64±0s                             |    0.69 | benchmark_imports.ImportBenchmark.time_from_spatialdata_io_import_xenium   |
| -        | 2.49±0.04s                 | 1.69±0.04s                          |    0.68 | benchmark_imports.ImportBenchmark.time_from_spatialdata_io_import_cosmx    |
| -        | 2.38±0.03s                 | 1.58±0.04s                          |    0.66 | benchmark_imports.ImportBenchmark.time_from_spatialdata_io_import_merscope |
| -        | 2.49±0.05s                 | 56.8±2ms                            |    0.02 | benchmark_imports.ImportBenchmark.time_import_spatialdata_io               |

Result from profimp below (maybe we could improve still a bit in a follow up PR):

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.39130% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.81%. Comparing base (7eb2e61) to head (609e40b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_io/__main__.py	24.13%	22 Missing ⚠️
src/spatialdata_io/readers/xenium.py	95.08%	3 Missing ⚠️
src/spatialdata_io/readers/_utils/_utils.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #373      +/-   ##
==========================================
- Coverage   62.70%   61.81%   -0.90%     
==========================================
  Files          27       27              
  Lines        3140     3153      +13     
==========================================
- Hits         1969     1949      -20     
- Misses       1171     1204      +33     

Files with missing lines	Coverage Δ
src/spatialdata_io/__init__.py	100.00% <100.00%> (ø)
src/spatialdata_io/readers/_utils/_image.py	100.00% <100.00%> (ø)
src/spatialdata_io/readers/generic.py	88.73% <100.00%> (-0.46%)	⬇️
src/spatialdata_io/readers/_utils/_utils.py	48.42% <66.66%> (-0.52%)	⬇️
src/spatialdata_io/readers/xenium.py	72.60% <95.08%> (+1.09%)	⬆️
src/spatialdata_io/__main__.py	81.42% <24.13%> (-5.81%)	⬇️

... and 2 files with indirect coverage changes

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