feat: add vcpkg support with proper CMake package config

[Copilot]

Fixes #26884

Description

Adds proper CMake installation infrastructure to enable vcpkg packaging. The repository previously only exported targets to the build tree, preventing vcpkg from creating a functional port.

Changes:

• CMakeLists.txt: Added install commands for library, headers, and CMake config files using GNUInstallDirs and CMakePackageConfigHelpers
• casbin/CMakeLists.txt: Changed include directories and dependencies from PRIVATE to PUBLIC with generator expressions for build/install interface separation
• cmake/casbinConfig.cmake.in: Created package config template that handles dependency resolution for nlohmann_json

Result:

Consumers can now use standard CMake discovery:

find_package(casbin REQUIRED)
target_link_libraries(myapp PRIVATE casbin::casbin)

The library installs to:

• ${CMAKE_INSTALL_LIBDIR}/casbin.a
• ${CMAKE_INSTALL_INCLUDEDIR}/casbin/
• ${CMAKE_INSTALL_LIBDIR}/cmake/casbin/

All 74 tests pass. vcpkg port can now be created using standard CMake build/install workflow.

Screenshots/Testimonials

N/A

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add casbin-cpp to vcpkg</issue_title>
<issue_description>### Description
I issued in vcpkg,but closed by someone.
Have a expert to reopen or Pull requests about casbin-cpp.</issue_description>

Comments on the Issue (you are @copilot in this section)

@nomeguy see: https://github.com/microsoft/vcpkg/pull/29027

• Fixes Add casbin-cpp to vcpkg #217

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