Implement loading feature for calibration & URDF files

[zhexuany]

Umbrella Issue: Implement Loading Feature for Calibration & URDF Files

This issue tracks the implementation of a flexible and robust loading feature for both calibration and URDF files, covering:

• Core loading functionality
• Fallback/automatic path discovery when no path is specified
• Attaching URDF files to MCAP attachments section
• Clear error reporting and user documentation

Dependencies

This issue is blocked by the following features:

• URDF as MCAP attachments (Feature: Add URDF as MCAP Attachments Section #30)
• Fallback loading rules and error reporting for blank calibration/URDF file paths (to be published)

Once both features are implemented, this umbrella issue can be closed. Please see referenced blocking issues for details, technical discussion, and implementation subtasks.

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Further sub-tasks and PRs should reference and close this issue as progress is made.