feat: preserve original point cloud fields and add optional downsampling for initial_map visualization

[gakutasu]

Summary

• initial_map was previously published as PointXYZI, discarding all
  original fields (e.g. RGB) from the source PCD/PLY file. It now uses
  raw_map_cloud (PCLPointCloud2) directly, preserving all fields.
• Added two parameters to control visualization quality vs. performance:
  • viz_downsample (bool, default: false): enable voxel downsampling
    for the published initial_map
  • viz_voxel_leaf_size (double, default: 0.5): leaf size used when
    viz_downsample is true

Motivation

Maps with RGB fields (e.g. colorized LiDAR or photogrammetry-derived PCD)
were not renderable in color in RViz2 because the conversion to PointXYZI
stripped the color data. Additionally, large maps caused RViz2 performance
issues due to publishing at full resolution.