Custom keyword support

Besides of standard keywords defined in json schema specification, library supports to create custom keyword for additional validation requirement. Eg:

{
  "type": "object",
  "properties": {
    "prop1": {
      "customKeyword": "Expected value"
    }
  }
}

ValidationKeywordRegistry.AddKeyword<CustomKeyword>();

[Keyword("customKeyword")] // It is your custom keyword name
[JsonConverter(typeof(CustomKeywordJsonConverter))] // Use 'CustomKeywordJsonConverter' to deserialize to 'CustomKeyword' instance out from json schema text
internal class CustomKeyword : KeywordBase
{
    private readonly string _customValue; // Simple example value

    public CustomKeyword(string customValue)
    {
        _customValue = customValue;
    }

    // Do your custom validation work here
    protected override ValidationResult ValidateCore(JsonInstanceElement instance, JsonSchemaOptions options)
    {
        if (instance.ValueKind != JsonValueKind.String)
        {
            return ValidationResult.ValidResult;
        }

        return instance.GetString() == _customValue
            ? ValidationResult.ValidResult
            : ValidationResult.CreateFailedResult(ResultCode.UnexpectedValue, "It is not my expected value.", options.ValidationPathStack, Name, instance.Location);
    }
}

internal class CustomKeywordJsonConverter : JsonConverter<CustomKeyword>
{
    // Library will input json value of your custom keyword: "customKeyword" to this method.
    public override CustomKeyword? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        // Briefly: 
        return new CustomKeyword(reader.GetString()!);
    }

    public override void Write(Utf8JsonWriter writer, CustomKeyword value, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }
}