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

## Problem

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

```csharp
[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:

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

Command:

```sh
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.

Start by locating the Newtonsoft C# renderer and the existing C# renderer-option and CLI-option handling. Run the reproduction command to capture current output, then verify that the opt-in mode changes optional reference annotations, Required handling, and FromJson's nullable result while the default output remains unchanged.

Written by the indexing model from the issue text.