Add support for `required` keyword in model properties

[bkoelman]

Model properties that use the required keyword (not to be confused with the [Required] attribute) don't always cause a validation failure when creating a new resource.

For example, in the following case, no validation error is produced when the name field does not appear in the POST request body:

public class TestModel : Identifiable<long>
{
    [Attr] public required string? Name { get; set; }
}

Addressing this would also fix the limitation described at https://www.jsonapi.net/usage/common-pitfalls.html#validation-of-required-value-types-doesnt-work.

The picture below shows the impact on JSON:API attributes. How this affects relationships is to be investigated.

The following model properties were used to create the picture above:

[Attr] public string NonNullableString { get; set; } = null!;
[Attr] public required string KeywordRequiredNonNullableString { get; set; }
[Attr] [Required] public string AttributeRequiredNonNullableString { get; set; } = null!;
[Attr] [Required] public required string KeywordAttributeRequiredNonNullableString { get; set; }

[Attr] public string? NullableString { get; set; } = null!;
[Attr] public required string? KeywordRequiredNullableString { get; set; }
[Attr] [Required] public string? AttributeRequiredNullableString { get; set; } = null!;
[Attr] [Required] public required string? KeywordAttributeRequiredNullableString { get; set; }

[Attr] public int NonNullableInt { get; set; }
[Attr] public required int KeywordRequiredNonNullableInt { get; set; }
[Attr] [Required] public int AttributeRequiredNonNullableInt { get; set; }
[Attr] [Required] public required int KeywordAttributeRequiredNonNullableInt { get; set; }

[Attr] public int? NullableInt { get; set; }
[Attr] public required int? KeywordRequiredNullableInt { get; set; }
[Attr] [Required] public int? AttributeRequiredNullableInt { get; set; }
[Attr] [Required] public required int? KeywordAttributeRequiredNullableInt { get; set; }

In this picture, the left-most block shows the behavior of a standard ASP.NET Web API project. The blocks to the right describe the JsonApiDotNetCore behavior for a POST/PATCH resource request. The blue markers indicate where a binding error should be produced (or the OpenAPI output needs to be adjusted) to resolve this issue.

Something to consider is the desired behavior when Model Validation and/or nullable reference types are turned off.