feat: option to use c# 11 required keyword for required properties

[alpoi-x]

This PR adds a UseRequiredKeyword option to the C# code generator. This will use the required modifier, added in C#11, for any properties that are required in the json schema.

[alpoi-x]

@RicoSuter ping!

[amis92]

I'd love this feature!

I was just looking how to do that myself. I do have an additional require-ment (jk), namely I want nullable properties (both structs and NRTs) to not have that keyword. I see that there's already a similar setting GenerateOptionalPropertiesAsNullable - would it be possible to also have a SkipRequiredKeywordOnNullableProperties with preferably better name, or an opposite of it? Pretty pretty please...

[alpoi-x]

@amis92 Isn't this already the case, given that a property is only treated as nullable if GenerateOptionalPropertiesAsNullable is enabled and the property is not required?

NJsonSchema/src/NJsonSchema.CodeGeneration.CSharp/Models/PropertyModel.cs

Lines 54 to 55 in 9bf8f69

	/// <summary>Gets a value indicating whether the property is nullable.</summary>
	public override bool IsNullable => (_settings.GenerateOptionalPropertiesAsNullable && !_property.IsRequired) || base.IsNullable;

[amis92]

That, or the null is allowed by type JSON Schema keyword.