Add null value and Setting type (#2478)

Author: swallez

specification/_spec_utils/utils.ts

@@ -0,0 +1,35 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import { Stringified } from '@spec_utils/Stringified'
+
+/**
+ * A `null` value that is to be interpreted as an actual value, unless other uses of `null` that are equivalent
+ * to a missing value. It is used for exemple in settings, where using the `NullValue` for a setting will reset
+ * it to its default value.
+ */
+type NullValue = null
+
+/**
+ * Settings in Elasticsearch are values that can be reset to their default by setting them to the `null` value.
+ *
+ * @es_quirk Because of how they are implemented internally, settings are always returned as strings, even
+ *           if their value has been set using a primitive type.
+ */
+type Setting<T> = Stringified<T> | NullValue