[Proposal] Add Python packaging support for SDFormat

## Desired behavior

SDFormat currently provides Python bindings but no standard way to build and distribute them as Python wheels on PyPI. The proposal is to enable creating a Python wheel using standard packaging tools to allow users to install SDFormat's Python bindings easily through a `pip install` command.

## Alternatives considered

Manual local builds and installs of Python bindings through CMake.

## Implementation suggestion

- Add a `pyproject.toml` and specify `scikit-build` as build backend to automatically produce Python wheels by calling `pip install .`
- Add a minimal `setup.py` to provide metadata and to ensure backwards compatibility.
- Modify `CMakeLists.txt` in `python` directory if necessary to expose proper targets for `scikit-build` to use
- Test and validate against multiple Python versions

## Additional context

- This enhancement will prepare the foundation for automating PyPI publishing though the existing Gazebo release pipeline (to be handled in a separate issue https://github.com/gazebo-tooling/release-tools/issues/1318)
- Testing can be initially done on TestPyPI before publishing official releases on PyPI.