Add IPv4/IPv6 preference support to ClientIpEnricher

[Copilot]

This PR implements IPv4/IPv6 preference functionality for the ClientIpEnricher to allow users to specify which IP version they want to log.

Changes Made

New IpVersionPreference enum

Added enum with the following options:

• None (default): No preference - use whatever IP version is available
• PreferIpv4: Prefer IPv4 addresses when multiple are available, fallback to IPv6
• PreferIpv6: Prefer IPv6 addresses when multiple are available, fallback to IPv4
• Ipv4Only: Only log IPv4 addresses, ignore IPv6 addresses
• Ipv6Only: Only log IPv6 addresses, ignore IPv4 addresses

Enhanced ClientIpEnricher

• Added IP version preference parameter to constructors
• Implemented filtering logic based on AddressFamily
• Maintains full backward compatibility

Extended Configuration API

Added new overload to support IP version preferences:

// Existing usage continues to work unchanged
.Enrich.WithClientIp()

// New usage with IP version preference
.Enrich.WithClientIp(IpVersionPreference.Ipv4Only)
.Enrich.WithClientIp(IpVersionPreference.Ipv6Only)

Example Usage

// Only log IPv4 addresses, ignore IPv6
Log.Logger = new LoggerConfiguration()
    .Enrich.WithClientIp(IpVersionPreference.Ipv4Only)
    .WriteTo.Console()
    .CreateLogger();

// Only log IPv6 addresses, ignore IPv4  
Log.Logger = new LoggerConfiguration()
    .Enrich.WithClientIp(IpVersionPreference.Ipv6Only)
    .WriteTo.Console()
    .CreateLogger();

Testing

• Added 15 comprehensive tests covering all preference scenarios
• Added 3 manual validation tests demonstrating real-world usage
• All existing tests continue to pass
• Verified backward compatibility

Documentation

• Updated README with usage examples and preference descriptions
• Added comprehensive XML documentation for new APIs

Backward Compatibility

✅ No breaking changes - all existing code continues to work without modification
✅ Default behavior remains unchanged
✅ New functionality is opt-in only

Fixes #48.

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