Add a [] Kotlin extension for PropertyResolver#getRequiredProperty

spring-core/src/main/kotlin/PropertyResolverExtensions.kt

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2002-2017 the original author or authors.
+ *
+ * Licensed 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.
+ */
+
+package org.springframework.core.env
+
+/**
+ * Extension for [PropertyResolver.getRequiredProperty] providing Array like getter.
+ *
+ * ```kotlin
+ * env["name"] = "Seb"
+ * ```
+ *
+ * @author Sebastien Deleuze
+ * @since 5.0
+ */
+operator fun PropertyResolver.get(key: String) : String = getRequiredProperty(key)