Add option to avoid nullability attributes from `JetBrains.Annotations` in generated C# models

### Problem
Currently, the C# model files [are generated with nullability attributes](https://github.com/JetBrains/rd/blob/53f0fb543891bdfe7d2577883d44175d60043765/rd-kt/rd-gen/src/main/kotlin/com/jetbrains/rd/generator/nova/csharp/CSharp50Generator.kt#L182) from `JetBrains.Annotations` (e.g., `[NotNull]`, `[CanBeNull]`). This creates an unwanted dependency: projects using the generated code must also depend on `JetBrains.Annotations`, which is not desirable in some cases.

### Proposed Solution
Add a generator option to skip nullability attributes from `JetBrains.Annotations` and use the `#nullable` compiler directive instead.