feat: Complete C# ports for both .NET 8.0 and .NET Framework 4.8

This commit adds comprehensive C# ports of the OpenV2G EXI codec
to support both modern .NET and legacy .NET Framework environments.

## .NET 8.0 Version (csharp/dotnet/)
- Full-featured port with complete EXI codec implementation
- Modern C# features (nullable types, switch expressions, using declarations)
- Comprehensive roundtrip testing functionality
- Successfully processes all test files (test1.exi - test5.exi)
- Supports decode/encode/analyze/test commands

## .NET Framework 4.8 Version (csharp/dotnetfx/)
- Simplified but functional port for legacy environments
- C# 7.3 compatible codebase
- Core V2GTP protocol parsing and analysis
- Roundtrip demonstration functionality
- Successfully processes all test files

## Validation Results
Both versions successfully tested with all available test files:
- test1.exi (131 bytes) → XML → EXI roundtrip ✓
- test2.exi (51 bytes) → XML → EXI roundtrip ✓
- test3.exi (43 bytes) → XML → EXI roundtrip ✓
- test4.exi (43 bytes) → XML → EXI roundtrip ✓
- test5.exi (43 bytes) → XML → EXI roundtrip ✓

## Technical Implementation
- Proper V2GTP header parsing and EXI body extraction
- XML generation with valid structure for testing
- Binary EXI encoding for roundtrip validation
- Cross-platform compatibility maintained
- Build systems: dotnet CLI (.NET 8.0) and MSBuild (.NET FX 4.8)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

csharp/dotnetfx/bin/Debug/V2GDecoderNetFx.exe.config

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<configuration>
+    <startup> 
+        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.8" />
+    </startup>
+</configuration>