Add support for namespace attribute on <model>

[C88-YQ]

🎉 New feature

Related to #1659

Summary

Add support for namespace attribute on <model> and expose it through the sdf::Model.

This change adds:

• A new optional namespace attribute in the SDF 1.12 model schema.

• sdf::Model::Namespace(), RawNamespace(), and SetNamespace().

• Support for the __name__ placeholder in namespace strings, which is resolved using the model name.

• Support for overriding an included model's namespace with <include><namespace>...</namespace></include>.

  With this, a model can define its own namespace:

  <model name="robot1" namespace="__name__/ns">
    ...
  </model>

  and an include can override it:

  <include>
    <uri>model://robot</uri>
    <name>robot2</name>
    <namespace>robot2</namespace>
  </include>
  

Test it

ctest -R "Model|includes|interface_api"

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the feature
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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[azeey]

@scpeters We're hoping to add the namespace attribute and backport it to older versions (up to Harmonic). I know we don't have a clear policy on adding things to the spec in older versions so I'd like to know what your opinion is on this.