feat(runtime): add DynamoField Format property for per-field format customization

[j-d-ha]

🚀 Pull Request

📋 Summary

This PR adds support for the Format property on the DynamoFieldAttribute, enabling per-field format string customization for formattable types (DateTime, DateTimeOffset, TimeSpan, Guid, and Enum). This allows individual properties to override the mapper-level format settings.

Key Changes

• Added Format property to DynamoFieldAttribute with comprehensive XML documentation
• Updated TypeMappingStrategyResolver to use field-level format when specified
• Added DynamoFieldFormatVerifyTests with comprehensive test coverage for:
  • DateTime with custom format (yyyy-MM-dd)
  • DateTimeOffset with custom format
  • TimeSpan with general long format (G)
  • Guid with N format (32 digits)
  • Enum with decimal format (D)
  • Multiple properties with different formats
  • Nullable types with format
  • Format override with mapper defaults
  • Non-formattable types (format is ignored)

✅ Checklist

• My changes build cleanly
• I've added/updated relevant tests
• I've added/updated documentation or README
• I've followed the coding style for this project
• I've tested the changes locally (if applicable)

🧪 Related Issues or PRs

Closes #51

💬 Notes for Reviewers

• The Format property only applies to formattable types (DateTime, DateTimeOffset, TimeSpan, Guid, Enum)
• For non-formattable types (string, bool, numeric types), the format is silently ignored
• Field-level format overrides mapper-level format (DateTimeFormat, TimeSpanFormat, GuidFormat, EnumFormat)
• All tests verify the generated code produces correct format strings in both ToItem and FromItem methods

[ncipollina]

✅ LGTM.