Add a toggle in getFitnessScore to respect user-provided indices in Registration

[AlrIsmail]

This PR resolves #6437 where Registration::getFitnessScore() completely ignored the indices_ member set via setIndices(). Previously, it evaluated the fitness score across the entire source cloud without a direct way to limit it.

Changes:

• Added an optional use_indices = false parameter to pcl::Registration::getFitnessScore.
• Updated the getFitnessScore implementation in impl/registration.hpp so that when use_indices = true and indices_ are present
• Added TEST(PCL, Registration_getFitnessScore_Indices) in test_registration.cpp

[mvieth]

Please fix the formatting by running clang-format (e.g. make format if you are on Linux)

[mvieth]

Thanks!