Add config option to specify 'discrete_contact_approximation'

See [here](https://drakedevelopers.slack.com/archives/C0JNB2E3S/p1705022059253719) for the Drake feature:
```python
 plant, scene_graph = AddMultibodyPlant(
        MultibodyPlantConfig(
            time_step=time_step,
            discrete_contact_approximation="similar"),
        builder)
```