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)
• codecheck passed (See contributing)
• 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.

[cheriehu]

The tests pass for me as well! Can I check if there's a corresponding gz-sim branch to test this with?

[C88-YQ]

@cheriehu Thanks for testing!

Yes, there is a corresponding gz-sim PR / branch for this work: gazebosim/gz-sim#3669

However, this branch currently includes changes for several related features, and we are planning to split it into several smaller PRs later to make the changes easier to review and test separately.

[C88-YQ]

Since there was some discussion around the placeholder syntax, I’ve started a Zulip thread to gather broader community input:
Zulip/Gazebo General/ SDFormat: choosing a placeholder syntax for namespace substi

I’ll update this PR once we’ve reached consensus on the placeholder syntax.