Detect unsupported union shapes in the code generator (#2012)

Union shapes of RdsDataService that have already been generated as
normal structure shapes are patched to keep support for them. Once
proper support for union shapes is implemented, they will be migrated to
union shapes in a major version of the package.

Author: stof

src/Definition/Shape.php

@@ -45,6 +45,8 @@ public static function create(string $name, array $data, \Closure $shapeLocator,
                     $shape = new ExceptionShape();
                 } elseif ($data['document'] ?? false) {
                     $shape = new DocumentShape();
+                } elseif ($data['union'] ?? false) {
+                    throw new \UnexpectedValueException(\sprintf('Union shapes are not supported yet by the code generator. The shape "%s" is a union shape.', $name));
                 } else {
                     $shape = new StructureShape();
                 }