Add schema references and extensible converters

[timkpaine]

Description

Adds independently resolvable provided and requested schemas with fsspec, Arrow IPC, structural JSON Schema, and Confluent Schema Registry providers. Schema credentials remain scoped to each reference and provenance is recorded before source data is opened.

Adds a native converter registry with lazy entry-point discovery plus optional Xarray-to-Zarr and Zarr-to-Xarray routes. Updates API, how-to, and explanation docs with the canonical data, schema, projection, codec, and native-converter flow.

Confluent support currently accepts JSON Schema documents without external references. Zarr remains a native multi-object resource conversion rather than a DataFileSystem single-file codec.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup
• CI / build configuration
• Other (describe below)

Checklist

• Linting passes (make lint)
• Tests pass (make test)
• New tests added for new functionality
• Documentation updated (if applicable)
• Changelog / version bump (not applicable; no release requested)

[github-actions]

Test Results

89 tests  +29   88 ✅ +28   0s ⏱️ ±0s
 1 suites ± 0    1 💤 + 1 
 1 files   ± 0    0 ❌ ± 0 

Results for commit 06c1e22. ± Comparison against base commit f2b7102.

[codecov]

Codecov Report

❌ Patch coverage is 84.95298% with 96 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.74%. Comparing base (f2b7102) to head (06c1e22).

Files with missing lines	Patch %	Lines
fsspec_data/confluent.py	64.04%	19 Missing and 13 partials ⚠️
fsspec_data/json_schema.py	73.33%	10 Missing and 10 partials ⚠️
fsspec_data/schema.py	90.20%	8 Missing and 6 partials ⚠️
fsspec_data/converters.py	83.78%	7 Missing and 5 partials ⚠️
fsspec_data/xarray_zarr.py	67.74%	5 Missing and 5 partials ⚠️
fsspec_data/tests/test_converters.py	92.00%	6 Missing ⚠️
fsspec_data/tests/test_schema.py	98.50%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
- Coverage   95.08%   89.74%   -5.34%     
==========================================
  Files          11       18       +7     
  Lines         631     1239     +608     
  Branches       30      102      +72     
==========================================
+ Hits          600     1112     +512     
- Misses         28       84      +56     
- Partials        3       43      +40     

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