Add RestClient Kotlin extensions

Closes gh-30807

spring-web/src/main/kotlin/org/springframework/web/client/RestClientExtensions.kt

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2002-2023 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
+ *
+ *      https://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.web.client
+
+import org.springframework.core.ParameterizedTypeReference
+import org.springframework.http.ResponseEntity
+
+/**
+ * Extension for [RestClient.RequestBodySpec.body] providing a `bodyWithType<Foo>(...)` variant
+ * leveraging Kotlin reified type parameters. This extension is not subject to type
+ * erasure and retains actual generic type arguments.
+ *
+ * @author Sebastien Deleuze
+ * @since 6.1
+ */
+inline fun <reified T : Any> RestClient.RequestBodySpec.bodyWithType(body: T): RestClient.RequestBodySpec =
+	body(body, object : ParameterizedTypeReference<T>() {})
+
+
+/**
+ * Extension for [RestClient.ResponseSpec.body] providing a `body<Foo>()` variant
+ * leveraging Kotlin reified type parameters. This extension is not subject to type
+ * erasure and retains actual generic type arguments.
+ *
+ * @author Sebastien Deleuze
+ * @since 6.1
+ */
+inline fun <reified T : Any> RestClient.ResponseSpec.body(): T? =
+	body(object : ParameterizedTypeReference<T>() {})
+
+
+/**
+ * Extension for [RestClient.ResponseSpec.toEntity] providing a `toEntity<Foo>()` variant
+ * leveraging Kotlin reified type parameters. This extension is not subject to type
+ * erasure and retains actual generic type arguments.
+ *
+ * @author Sebastien Deleuze
+ * @since 6.1
+ */
+inline fun <reified T : Any> RestClient.ResponseSpec.toEntity(): ResponseEntity<T> =
+	toEntity(object : ParameterizedTypeReference<T>() {})