Added OneOf to built-in types and well-known directives

Author: glen-84

src/HotChocolate/Core/src/Types.Shared/BuiltInTypes.cs

@@ -28,7 +28,8 @@ public static class BuiltInTypes
         WellKnownDirectives.Deprecated,
         WellKnownDirectives.Defer,
         WellKnownDirectives.Stream,
-        WellKnownDirectives.SpecifiedBy
+        WellKnownDirectives.SpecifiedBy,
+        WellKnownDirectives.OneOf
     ];
 
     public static bool IsBuiltInType(string name)

src/HotChocolate/Core/src/Types.Shared/WellKnownDirectives.cs

@@ -9,4 +9,5 @@ internal static class WellKnownDirectives
     public const string Deprecated = "deprecated";
     public const string SpecifiedBy = "specifiedBy";
     public const string DeprecationReasonArgument = "reason";
+    public const string OneOf = "oneOf";
 }