[Feature request] http-server-csharp: Configuration key to turn of generating Operations controller

### Clear and concise description of the problem

My RP is behind RPaaS so doesn't need its own Operations controllers, RPaaS will handle it for me.

I'd like to have an option to disable generating it altogether.

### 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.