Add basic DD4HEP integration

[sethrj]

• [ ] Add DD4HEP as an optional cmake dependency
• [ ] Add DD4HEP to spack CI infrastructure (@sethrj )
• [ ] New subdirectory src/ddceler that contains dd4hep_add_plugin
• [ ] Physics list for adding tracking manager
• [ ] Run action for setup/teardown
• [ ] Example showing minimal use through dd4hep execution