Add a CoExchangeFilterFunction for WebClient

This commit introduces a Kotlin-specific implementation
of the ExchangeFilterFunction interface that allows for
using coroutines, similar to what CoWebFilter does on
server-side.

Closes gh-30650

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/client/CoExchangeFilterFunctionTests.kt

@@ -0,0 +1,48 @@
+/*
+ * 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.reactive.function.client
+
+import io.mockk.mockk
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.runBlocking
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+
+/**
+ * Test for [CoExchangeFilterFunction].
+ *
+ * @author Sebastien Deleuze
+ */
+class CoExchangeFilterFunctionTests {
+
+	@Test
+	fun exchange() {
+		val response = mockk<ClientResponse>()
+		val exchangeFunction = MyCoExchangeFunction(response)
+		runBlocking {
+			assertThat(exchangeFunction.exchange(mockk())).isEqualTo(response)
+		}
+	}
+}
+
+private class MyCoExchangeFunction(private val response: ClientResponse) : CoExchangeFunction {
+
+	override suspend fun exchange(request: ClientRequest): ClientResponse {
+		delay(1)
+		return response
+	}
+}