C#: add an opt-in Newtonsoft-compatible nullable reference output mode

[foobra]

Problem

When generating C# models for an API whose object properties are optional, the Newtonsoft renderer can emit properties such as:

[JsonProperty("profile", Required = Required.DisallowNull, NullValueHandling = NullValueHandling.Ignore)]
public Profile Profile { get; set; }

This is awkward for consumers using nullable reference types: the property can be absent from JSON, so its default value after deserialization is null, but the generated C# declaration is non-nullable. With --check-required, an explicit JSON null can also be rejected by Newtonsoft.Json.

The generated FromJson helper also returns a non-nullable model and directly propagates deserialization exceptions.

Reproduction

Schema:

{
  "title": "Request",
  "type": "object",
  "properties": {
    "profile": {
      "type": "object",
      "properties": {
        "id": { "type": "string" }
      }
    },
    "displayName": { "type": "string" }
  }
}

Command:

quicktype \
  --src request.schema.json \
  --src-lang schema \
  --lang csharp \
  --framework NewtonSoft \
  --csharp-version 6 \
  --features complete \
  --check-required \
  --out Request.cs

Requested behavior

Please add an opt-in renderer option, without changing existing defaults, for consumers that want this compatibility behavior:

• enable nullable reference annotations for optional reference properties;
• avoid emitting Required.DisallowNull for properties that may be absent;
• make the top-level FromJson helper's nullable result explicit;
• preserve the current output when the option is not supplied.

A possible name is --csharp-nullable-reference-types or another name that fits the existing option model.