Add Loosely Coupled GNSS Integration to Kimera-VIO

[kleo-53]

Feature

This pull request adds loosely coupled GNSS support to Kimera-VIO. GNSS measurements are integrated as position-only factors in the backend using custom GTSAM factors.

Motivation

To improve trajectory estimation by incorporating GNSS data into the stereo visual-inertial pipeline, especially in large-scale or degraded-vision scenarios.

Implementation

• Adds GNSS data provider and buffer
• Extends frontend/backend to handle GNSS
• Injects GNSS factors into graph via BeforeOptimizeHook()
• Provides more accurate trajectory then stereo vio pipeline

Notes

Developed as part of a Bachelor's thesis at SPBU. Evaluation done on synthetic GNSS data derived from EuRoC ground truth.