[http-server-csharp] Allow user to specify attributes that are emitted.

### Clear and concise description of the problem

C# model classes require attributes, especially when integrating with Entity Framework.

In my final C# class I need, JsonIgnore, or NotMapped attributes, or other attributes as I see fit for my model classes to integrate with other parts of my system.

i.e. JsonIgnore example here:

```csharp
[JsonIgnore]
public ThingGroup? ThingGroup { get; set; }
```

I would like to be able to specify those attributes in my tsp files.

### 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.