feat(cpp): improve CMake configuration for portable distribution and installation (#810)

[shivendra-dev54]

Reason for this PR

This PR addresses #810 to improve the portability and installation of the GraphAr C++ library. The current setup makes it difficult for downstream projects to use find_package(graphar), which this update resolves by modernizing the CMake export logic.

What changes are included in this PR?

• Modern CMake Targets: Added support for the graphar::graphar namespace via install(EXPORT).
• Hybrid Linking Strategy: Updated dependencies (Arrow/Parquet) to PUBLIC visibility to ensure proper propagation to consumers.
• Relocatable Installation: - Integrated GNUInstallDirs for standard-compliant paths.
  • Set CMAKE_INSTALL_RPATH using $ORIGIN (Linux) and @loader_path (macOS) to ensure the library is relocatable.
• Config File Modernization: Updated graphar-config.in.cmake using @PACKAGE_INIT@ and find_dependency for robust downstream resolution.
• Backward Compatibility: Preserved the legacy GRAPHAR_LIBRARIES and GRAPHAR_INCLUDE_DIR variables.

Are these changes tested?

• Verified that all 20 existing C++ unit tests pass (ctest).
• Confirmed that make install generates a valid relocatable file structure.
• Successfully linked a separate "Hello World" project using find_package(graphar REQUIRED).

Are there any user-facing changes?

Yes. Users can now integrate GraphAr into their own projects using the standard CMake pattern:

find_package(graphar REQUIRED)
target_link_libraries(my_app PRIVATE graphar::graphar)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.86%. Comparing base (d535a43) to head (28d3f6f).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #890      +/-   ##
============================================
+ Coverage     79.81%   79.86%   +0.04%     
  Complexity      615      615              
============================================
  Files            93       93              
  Lines         10296    10296              
  Branches       1055     1055              
============================================
+ Hits           8218     8223       +5     
+ Misses         1838     1833       -5     
  Partials        240      240              

Flag	Coverage Δ
cpp	70.89% <ø> (+0.09%)	⬆️

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[shivendra-dev54]

@yangxk1
Could you please review and confirm whether this meets the requirements of the issue?

[yangxk1]

cc @Sober7135