Add per-marker size configuration for STag pose estimation

[RokPre]

This pull request adds support for using different physical sizes for different STag IDs.

Previously, stag_detect assumed a single global marker_size for all detected markers. That worked only
when every marker in the environment had the same printed size. With this change, the node can
optionally load a YAML config file that maps marker IDs to sizes, and use the configured size for pose
estimation on a per-marker basis.

This allows mixed deployments such as:

• larger markers for long-range detection
• smaller markers for close-range detection

Implementation summary:

• adds a new config_file parameter to stag_detect
• loads marker sizes from YAML at startup into an id -> size map
• uses the configured size for each detected marker when building the marker’s 3D corner model before
  pose estimation
• keeps marker_size as the fallback when a marker ID is not listed in the config
• uses a square-marker PnP path for pose estimation

Example config:

default_marker_size: 0.175
markers:
- id: 0
size: 0.175
- id: 5
size: 0.09
- id: 6
size: 0.09

Example launch usage:

ros2 launch stag_detect stag_detect.launch.py config_file:=/path/to/marker_sizes.yaml