feat: implement dataclasses support for voluptuous schemas

[doubledare704]

Resolves GitHub issue #409 by adding comprehensive dataclasses support:

Features:

• DataclassSchema class for automatic schema creation from dataclasses
• create_dataclass_schema() function for functional API
• is_dataclass() utility for dataclass detection
• Automatic type validation from dataclass field annotations
• Support for default values and default_factory functions
• Seamless integration with existing voluptuous validators
• Type annotation handling for List[T], Optional[T], etc.
• Full backward compatibility with existing schemas

Implementation:

• Added voluptuous/dataclasses_support.py with core functionality
• Added comprehensive test suite (19 tests covering all scenarios)
• Added examples/dataclasses_example.py with usage examples
• Updated README.md with documentation and examples
• Integrated into main voluptuous module with graceful fallback

Code Quality:

• All tests pass (162 existing + 19 new = 181 total)
• Zero flake8 violations
• Zero mypy type errors
• Black and isort formatting applied
• Requires Python 3.7+ for dataclasses support

Breaking Changes: None
Backward Compatibility: Full