Support [DataMember] IsRequired in NewtonsoftDataContractResolver

[ouvreboite]

Resolves #2383

• Add non-regression tests to ensure [DataMember(IsRequired)] do not bypass existing [JsonObject], [JsonProperty] or [JsonRequired] attributes.
• Add tests for [DataMember(IsRequired)] and [DataMember(name)]
• Update NewtonsoftDataContractResolver to directly leverage jsonProperty.Required, instead of using the IsRequiredSpecifed extension method