Plane: Implement command for setting relative altitude to HAGL based on rangefinder measurements

[rubenp02]

Work in progress implementation of a functionality to set the aircraft's relative altitude using a robust HAGL estimate derived from rangefinder readings. The goal is to mitigate barometric drift during long flights without requiring a full rangefinder-based landing, which can be impractical over uneven terrain or areas with significant obstacles along the glide slope. It is based on #29266, allowing the rangefinder to only activate as the aircraft reaches the landing point while ensuring a highly precise approach altitude if this command is triggered over the landing area beforehand.