feat: add support for user-defined custom metadata from ND acquisition

[jmarkow]

We are using custom metadata when acquiring data via ND acquisition in Nikon Elements (AR 6.02.01 64-bit). We could not find the metadata in any of the commonly used metadata fields in ND2File. After poking around, we found the metadata lived in CustomData|CustomDescriptionV1_0!. Using json_from_clx_lite_variant without modification only loaded in a subset of the metadata. This turned out to be due to keys getting clobbered by https://github.com/tlambert03/nd2/blob/main/src/nd2/_parse/_clx_lite.py#L234 . This PR includes a minimal set of changes to safely load in custom metadata under the custom_metadata field. We also included an opt-in modification to the clx parser to prevent keys from getting clobbered.

Let us know if you need anything else here.

[codspeed-hq]

Merging this PR will not alter performance

✅ 13 untouched benchmarks

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_{Comparing markolab:nd-acq-custom-metadata-fix (cdf292d) with main (37b131f)}

[codecov]

Codecov Report

❌ Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.83%. Comparing base (37b131f) to head (cdf292d).

Files with missing lines	Patch %	Lines
src/nd2/_readers/_modern/modern_reader.py	33.33%	4 Missing ⚠️
src/nd2/_nd2file.py	66.66%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (54.54%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #300      +/-   ##
==========================================
- Coverage   93.63%   89.83%   -3.81%     
==========================================
  Files          22       22              
  Lines        2607     2616       +9     
==========================================
- Hits         2441     2350      -91     
- Misses        166      266     +100     

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