Parse and set bullet solver iterations

[iche033]

🎉 New feature

Summary

Added support for users to configure the bullet solver iterations.

It parses the //world/physics/bullet/solver/iters SDF param and sets that value to the physics engine using the new Solver feature, SetSolversIterations, introduced in https://github.com/gazebosim/gz-physics/pull/609.

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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