[http-server-csharp] Allow user to specify a custom namespace

### Clear and concise description of the problem

Right now the namespace for model, operations, etc matches what is in the tsp files. But in some cases the user is going to want to include these generated files in a different project with a different namespace. It would be nice if they could specify a namespace of their choosing.

### 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.