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

[garidozv]

Problem

Currently, the C# model files are generated with nullability attributes 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.