Prototype system for initializing model state

[scpeters]

🎉 New feature

Closes #2318

Summary

This adds a system called SetModelState that accepts an xml configuration containing <model_state> tags intending to serve as a prototype for new syntax for the <state> tag in SDFormat 1.12. Currently, only <joint_state> tags are supported; support should also be added for <link_state>.

Example <model_state> syntax from examples/worlds/set_model_state.sdf

• Using degrees

        <model_state>
          <joint_state name="j1">
            <axis_state>
              <position degrees="true">60</position>
              <velocity degrees="true">-30</velocity>
            </axis_state>
          </joint_state>
        </model_state>

• Using radians

        <model_state>
          <joint_state name="j1">
            <axis_state>
              <position>1.047</position>
              <velocity>-0.523</velocity>
            </axis_state>
          </joint_state>
        </model_state>

Test it

gz sim -v 4 examples/worlds/set_model_state.sdf

The rotors should start at a 60 degree angle and rotate clock-wise at 30 degrees / second.

Checklist

• [ ] Signed all commits for DCO
• [ ] 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)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

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