@typespec/http-server-csharp: Generate `JsonPolymorphicAttribute`

### Clear and concise description of the problem

For models with `@discriminator` or discriminated unions we should generate the `JsonPolymorphicAttribute` and `JsonDerivedTypeAttribute` to make serialization work correctly. This would require omitting the discriminator property in the generated class though (It's generated by the JSON serializer).

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.