[csharp] Added x-nullable-value-type

[devhl-labs]

Adds x-nullable-value-type which is true if the isNullable == true and x-is-value-type == true
Adds x-nullable-type if nrt is on or if the object is a nullable value type.

Here is the documentation on nullable value types. The tldr is that a nullable value type is really a class of type Nullable<T> where T is a value type.
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/nullable-value-types

In another pr I plan on

• removing the ? that is hard coded into data types
• removing x-csharp-value-type
• changing x-is-value-type to only return true for value types
• removing nullableType
• adding more value types to valueTypes

@mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05)

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[wing328]

removing the ? that is hard coded into data types

i'm ok with that but previously some users want to have the ? hard coded as they don't want to add nullable to the schema definition one by one.

if that's still an use case, we can then add a rule in the openapi normalizer to add nullable: true to all schemas in the spec.

[devhl-labs]

You're talking about whether isNullable is true. Removing the hard coded ? from the data type is not related. If anyone wants all schemas to be nullable then the normalizer seems like a good idea, though it kind of pains me that the spec is incorrect. This pr and the plan for the next should not really be related though.