Add multi-platform support for pyarmor_runtime executables
1. Platform Detection and Executable Selection:
- Automatically detects OS (Windows, Linux, macOS) and architecture
- Intelligently selects the appropriate executable based on platform
- Supports architecture-specific executables (e.g., pyarmor-1shot-windows-x86_64.exe)
- Falls back to platform-specific and generic executables when needed
2. Runtime File Auto-Detection:
- Added ability to automatically find all pyarmor_runtime files in a directory
- Supports multiple runtime file formats (.pyd, .so, .dylib)
- Recursively searches through all subdirectories
3. New Command-Line Options:
- Added --executable option to specify a custom executable path
- Added --auto-detect-runtime option to automatically detect runtime files
