Introduce CoWebFilter

This commit introduces the CoWebFilter, an abstract WebFilter
implementation that allows for coroutine usage.

Closes gh-29672

spring-web/src/test/kotlin/org/springframework/web/server/CoWebFilterTests.kt

@@ -0,0 +1,56 @@
+/*
+ * 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.server
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.mockito.BDDMockito
+import org.mockito.Mockito
+import org.springframework.web.testfixture.http.server.reactive.MockServerHttpRequest
+import org.springframework.web.testfixture.server.MockServerWebExchange
+import reactor.core.publisher.Mono
+import reactor.test.StepVerifier
+
+/**
+ * @author Arjen Poutsma
+ */
+class CoWebFilterTests {
+
+	@Test
+	fun filter() {
+		val exchange = MockServerWebExchange.from(MockServerHttpRequest.get("https://example.com"))
+
+		val chain = Mockito.mock(WebFilterChain::class.java)
+		BDDMockito.given(chain.filter(exchange)).willReturn(Mono.empty())
+
+		val filter = MyCoWebFilter()
+		val result = filter.filter(exchange, chain)
+
+		StepVerifier.create(result)
+			.verifyComplete()
+
+		assertThat(exchange.attributes["foo"]).isEqualTo("bar")
+	}
+}
+
+
+private class MyCoWebFilter : CoWebFilter() {
+	override suspend fun filter(exchange: ServerWebExchange, chain: CoWebFilterChain) {
+		exchange.attributes["foo"] = "bar"
+		chain.filter(exchange)
+	}
+}