Add XML schema validation for ros2_control tag and test assets

[sachinkum0009]

Description

Note: This PR is a recreation of the accidentally closed PR #2466.

It introduces XML schema validation for the <ros2_control> tag in URDF files, addressing Issue 2173. The changes include:

• Added an XML Schema Definition (ros2_control.xsd) to formalize and validate the structure of the <ros2_control> tag.
• Implemented a GTest-based unit test (validate_xml_schema.cpp) using libxml2 to validate URDF files against the schema.
• Updated CMakeLists.txt to install the schema and URDF directories, and to build and link the new test.
• Added a new invalid URDF test file to verify schema validation failure cases.
• Enhanced the valid test URDF to include more schema features (e.g., params, gpio).
• Updated package dependencies to include ament_index_cpp for test support.

Fixes #2173

Is this user-facing behavior change?

Did you use Generative AI?

No

Additional Information

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