Improved JsonApiClient and relationships into `openapi-required-and-nullable`#1231

maurei merged 7 commits intoopenapi-required-and-nullable-propertiesjson-api-dotnet/JsonApiDotNetCore:openapi-required-and-nullable-propertiesfrom

attributes-object-schema-property-testsjson-api-dotnet/JsonApiDotNetCore:attributes-object-schema-property-testsCopy head branch name to clipboard