fix(launch): resolve Pydantic v2 compatibility and update build instructions

[jwsung91]

Description

This Pull Request addresses two main issues related to building and launching the usb_cam package on newer environments (specifically ROS 2 Jazzy on Ubuntu 24.04):

1. Pydantic v2 Compatibility:

   • ROS 2 Jazzy uses Pydantic v2, which deprecates/breaks the older Pydantic v1 validator syntax (@root_validator) used in launch/camera_config.py.
   • Updated the imports in launch/camera_config.py to import from pydantic.v1 with a fallback to pydantic. This maintains backward compatibility with ROS 2 Humble/Pydantic v1 while fixing the crash on ROS 2 Jazzy.

2. Build and Sourcing Documentation:

   • Updated README.md to explicitly guide users through sourcing their ROS 2 installation.
   • Added steps for installing and initializing rosdep if it is missing on the system.
   • Updated dependency installation commands to dynamically use ${ROS_DISTRO} for convenience.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Documentation update

How Has This Been Tested?

• Tested building under ROS 2 Jazzy with a clean workspace (colcon build).
• Verified that ros2 launch usb_cam camera.launch.py starts correctly without throwing Pydantic-related Python exceptions.