feat: add HiveConverter for custom TypeAdapter field conversion

[Rexios80]

Summary

Adds a json_serializable-style type converter API so third-party types can be handled without hardcoding them into the generator (see #286).

User API

class UriConverter implements HiveConverter<Uri, String> {
  const UriConverter();

  @override
  Uri fromHive(String hive) => Uri.parse(hive);

  @override
  String toHive(Uri object) => object.toString();
}

@GenerateAdapters(
  [AdapterSpec<Website>()],
  converters: [UriConverter()],
)
class Website {
  final Uri url;
}

Converters are registered only via GenerateAdapters.converters. Matching is by field type (including simple generic converters like IListConverter<T> → IList<String>).

Design notes

• User-facing shape mirrors JsonConverter (fromHive / toHive)
• Implementation is intentionally small: no field/class annotations, no AdapterSpec.converters
• Nested collections apply converters to elements on read/write

Test plan

• GenerateAdapters.converters generation
• Generic converter type-argument inference
• Nested collection conversion
• Nullable fields
• Example project with UriConverter
• CI green

  

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.37%. Comparing base (e98c319) to head (f0df1c6).

Files with missing lines	Patch %	Lines
hive/lib/src/annotations/hive_converter.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #313      +/-   ##
==========================================
- Coverage   93.40%   93.37%   -0.04%     
==========================================
  Files          64       65       +1     
  Lines        2625     2626       +1     
==========================================
  Hits         2452     2452              
- Misses        173      174       +1     

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