Cherry pick training utils into phase_1

[kaushikbalasundar]

PR Description: Add xacro expansion service for training

This PR cherry-picks the ExpandXacro service and related training utilities to the phase_1 branch.

Key Changes:

• Added ExpandXacro ROS 2 service in aic_utils/aic_training_interfaces.
• This service allows model-side code to request xacro-to-XML expansion without requiring direct filesystem access to the eval workspace's package share directories.
• Added xacro_expander.py script and related launch files in aic_utils/aic_training_utils.

Motivation:

To support training workflows where model-side code needs to dynamically expand Xacro files.

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