[http-client-csharp] Supporting option for generating the client libraires with safe support of nullable reference types

### Clear and concise description of the problem

Currently the code generated disables nullable reference types (adding `#nullable disable` to every file) and accordingly it does not mark optional properties or parameters as nullable. As a result, the client libraries generated can't be used in a safe way in codebases with nullability support enabled.

It would be beneficial to support an emitter option for generating code that respect nullable reference types ensuring the client libraries users who are leveraging this language feature, can use the client libraries generated safely too.

### 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.